I use a UUID custom function for my table's key fields. I haven't encountered a situation where the user needs to know the ID.

I notice that the calendar demos show a prefixed ID.

Is there a problem with IDs that I will encounter?

No. We used prefixed IDs because it is/was easier to let people know what table they were looking at, but we don't use the prefixing for any functionality in the Calendar or Complete. Since GoZync we're using UUIDs in most of our new work now.
So you're just ahead of the curve. ;-)

"So you're just ahead of the curve. " No one ever accused me of being there before! <GRIN>

Flattery will get you everywhere! Thanks!

I've had plenty of problems with the FM serial number for an ID. Its too easy for me to lose sync with records while importing and exporting.

Actually, I follow a lot of Matt Petrowsky's stuff at http://www.filemakermagazine.com/ and http://filemakerstandards.org/display/cs/Overview .