Now we're speaking my language.

I'd hope so, but that kind of goes against what RFID does. You interrogate the chip, it responds with its ID....SKU for inventory, employee ID for a badge, that kind of thing. The end-chips are pretty stupid, for lack of a better term. They don't process, they don't compute....embedding processors into chips (think a wireless smart card) will add a SIGNIFICANT cost to the whole implementation. Think dollars per bracelet compared to pennies.....even with Disney's economy of scale.
There have been some papers written on the subject of authentication. I think one was called "Forward-secure RFID Authentication and Key Exchange" but I haven't heard anything written about a real-world implementation. Talk about a test-case.
Just encrypting the information stored on the bracelets wouldn't do much good. You could skim just the same and then pass that encrypted value to the reader; just like a pass-the-hash password attack. Sure, you might not get access to the real data (credit card, room number, names), but it doesn't much matter.
Also, don't think so fast that Disney wouldn't roll out something insecure. Many of the kiosks that take credit cards/KTTW in the park are using WiFi for communication and (if memory serves, its been a while since I fired up a WiFi analyzer on my phone) they were using encryption methods easily bypassed/intercepted. Not saying it's happening, but it could.