Why is my Magic TrackPad losing functionality after Fast User Switching several times?
Okay, may have found something, posted to my other thread on this topic: https://discussions.apple.com/message/15963083#15963083

Okay, some potentially good news:

Under System Preferences > Bluetooth > Advanced > option to Allow Bluetooth devices to wake this computer

This is turned on by default and is apparently a per-user setting, not a global setting, which is where the conflict begins, from what I can tell.

My guess is that, when turned on for all users, like it is by default, when the computer goes to sleep, the bluetooth signal gets screwy, probably stepping over itself, to listen for incoming requests for both users for both keyboard and trackpad. And/or, when a user wakes the computer by pressing one of these devices, the signal tries to relaunch the Bluetooth stack for both users for both devices immediately, again probably stepping over itself, setting up a chain of events that will cause Bluetooth connectivity to fail altogether after a few sleep/wake and/or Fast User Switching cycles.

So, for a couple days now I've been running with this option turned off for one user and on for the other user. So, depending on which user is currently logged into the console, you may have to wake the computer by way of the power button or the keyboard/trackpad. The good news, though, is that it would typically foul up after no more than a day, and it's gone a couple of days thus far without issue.