If there was a, "hidden," cap on an advertised unlimited data plan, I'm positive that it would be mentioned somewhere at some point. Verizon can be a bunch of greedy bastards sometimes, but I don't think that even they would go that far.
Someone should actually call up a Verizon rep. and ask this very question. It can't hurt to ask the source, can it? IMHO, worst case scenario?: You call them up and ask them about any potential download cap on your unlimited data plan. They tell you that such a thing doesn't exist, but might make a note on your account. If such a hidden charge actually exists and you go over 5GB in a month, they'll see the note and not actually charge you for it. :> The last thing they need is a customer calling and complaining that they were specifically told there wouldn't be any hidden data charges on they're supposedly, "unlimited," data plan, then getting charged anyway. Though, I'm just speculating/babbling.
So far I've used about 2.6GB on my plan and I don't think I've had my LG Ally for even a week yet. My plan rolls over on the 18th, so if I've gone over 5GB by then, I'll be sure to let everyone know if any miscellaneous charges show up on my bill.

I will most likely have my Droid X by then as well, so I should be even more capable of making that mark by that deadline.