I'll put off the backhanded commentary that I was affecting for the sake of bantering with Tarkaan and address this question squarely. In my Scripture study recently I've been paying particular attention to the concept of "the rest of the Lord", which is a surprisingly deep topic. What I'm learning sheds surprising light on this particular question.
When reading the Chronicles, an important principle to keep in mind is what I call "the Aslan Effect" - i.e. watch for when Aslan appears in the story and attend to what He does, because it will illuminate key themes of the tale. For instance, Aslan's role in Lion is more obvious: His sacrificial death on the Stone Table and His resurrection are clearly meaningful, as is His resuscitation of the ossified Narnians and His triumph over the Witch at Beruna.
In Caspian, His appearances are less clear. He appears to Lucy in what you might call a special revelation, and then commissions and empowers her for a difficult task: getting the others moving on the proper path. He does not make Himself immediately clear to the others (a point which Peter uses as an argument against believing); they have to take His presence on faith and upon the word of the prophetess Lucy. The more they obey the more clear their vision becomes, and they begin to see Aslan starting with the most willing to believe (Edmund) and ending with the most fearful (Susan) and unbelieving (Trumpkin, who is also fearful in his own way.) This little byplay, which happens primarily at the beginning of the chapter The Lion Roars, illustrates the theme of obedience in spite of external appearances.
Following this is Aslan's Big Appearance in Caspian - which is, suprisingly, not the establishment of King Caspian, the risky and dangerous single combat with the tyrant Miraz, or the Second Battle of Beruna. While all these significant and critical events are happening, where is Aslan? That's the vital question, for where Aslan is will be the heart of the true action, the truly Significant and Critical Events.
As it turns out, Aslan is sponsoring a party. Leaving his lieutenants to deal with the lesser matters of a difficult and dangerous battle (which they come very close to losing), He is romping about the land with the queens, liberating everything in sight. And that, I think, is the central idea: liberation. Aslan and his crew are a little wild to begin with (though not uncontrolled, as Lucy and Susan observe), and wherever they frolic, the immediate result is liberation from bondage and slavery. Look at the vignettes: the River God is loosed from his chains (the Beruna Bridge), the student Gwendolen who welcomes Aslan is liberated from her bondage in her class (and her stiff clothing in the bargain), chained dogs broke their chains, carthorses kicked their carts to pieces, a boy is freed from an oppressive master (who is turned into a tree), a schoolmistress is freed from the tedium of her job, and Caspian's old nurse is given reprieve from the bonds of old age and impending death.
Do we see a pattern here? I'm convinced that what we're witnessing is a Narnian Sabbath, or perhaps more accurately a Narnian Jubilee (Leviticus 25) - the Rest of Aslan. Not only is there no work (toil) done, but the very instruments of toil (chains, rods, schoolrooms) are broken and destroyed. The only "work" is to rejoice and revel in Aslan's presence, to set free those in bondage, to welcome the captives into a new day of freedom, to feast and rejoice and fellowship with friends old and new. After ten generations of enslavement to their Telmarine conquerors, the Narnians are set free to be who they are: the Great River flows unhindered, the beasts (dumb and Talking) are again free to simply live rather than justify their existence by working.
Of particular significance is the Talking Trees. Remember Aslan's commission to the Narnians at the beginning: "Narnia, Narnia, Narnia, awake." His particular command to the trees was, "Be walking trees." In response to the Telmarine oppression, the trees had ceased walking and fallen back asleep - essentially doing the reverse of what they'd been commanded. But when Aslan returns proclaiming liberty, they respond and take up their assigned role.
Where we find Aslan, we find the center of the action. This is the central theme, the Really Important Message of the entire book. I am not surprised that certain members even in this thread find the Frolic opaque or even ridiculous and contemptible - to the natural mind, it seems like this is a frivolous distraction from the truly important events, which in Caspian would be the struggle for political power over Narnia and the battle. (In my opinion this is where the filmmakers went wrong - they made that the exclusive focus, turning it into a comic book event that totally misses the point.) But in the book the seemingly important events are almost peripheral: Peter's combat with Miraz proves inconclusive, and even with the presence of the Ancient Heroes the outcome of the battle is in doubt until...
Until what? Until an event that was completely out of their hands: the charge of the Trees. That was instigated by Aslan's liberation of them, and none of the doing of the warriors. They had to "hold the fort" - as Peter puts it, "do what we can on our own" - which they did with determination, courage, and no small amount of risk. But the final resolution of the battle is beyond them (to me, an excellent illustration of the principle of Isaiah 30:15), and the Telmarines are finally defeated not by the might of Caspian's army but by the silent power of the liberated Narnians - most particularly the woods and waters. Notice there's no indication that the Trees actually harmed anyone (another thing the filmmakers got wrong) - the fear of them was sufficient to knock the fight out of the Telmarines.
So it seems to me that the Frolic was not simply a pivotal event in Caspian, but a central theme of the entire Chronicles. Narnia is the Sacramental Land, the Rest of Aslan, to the Narnian universe. It was created directly by fiat of Aslan, it is the center and hub of all Aslan's work, through Narnia flows all Aslan's grace to all the peripheral lands. Narnians work to be sure, but it is the willing work of free citizens, not the grinding toil of an enslaved populace. They live in the Rest of Aslan - who would not celebrate?
