Here's what I found over the last several weeks of using Lion: if my wife and I both made a point to close iPhoto before walking away from the computer or before the other person Fast-User-Switched over to their profile, the problem seemed to go away. I suspect iPhoto used the discrete/accelerated graphics chip, though I have no idea if that's related. Back in the day, I thought I'd noticed the same thing happening if I left Sparrow or Twitterrific open on my profile before my wife FUS'd over to her profile.
Now, we've upgraded to Mountain Lion, and I'm scared to even experiment just yet, but we've continued to be religous about keeping iPhoto closed. In the coming days and weeks on ML, we'll attempt leaving iPhoto open or maybe turning "allow bluetooth devices to wake this computer" back on and see what happens. Oh how awesome it would be to actually be able to utilize features like that and/or not worry so much about leaving iPhoto or other 3rd party apps up and running before the other user logged in!
I'll keep everyone posted as I learned anything new (in case anyone still cares about this problem).
I have found that Mountan Lion 10.8 seems to have largely fixed these problems for me. My wife and I have ceased worrying about whether iPhoto is left open or not on either of our profiles, and the problem has yet to rear its head.
The next step is to again try enabling "allow bluetooth devices to wake computer" to see if that's now fixed, too.
I'm having the same issue on Mountain Lion 10.8.2. I do not have iPhoto, for me simply clicking "Switch Account" from the lock screen causes the Magic Trackpad to stop tracking. I have plugged in another USB mouse so I can post this reply, the weird thing is if I click my Magic Trackpad, I can click just fine, so it isn't like the Trackpad lost connection, I just can't move the mouse cursor - This is very annoying and forces me to have to reboot every time this happens.
I pretty much don't have the problem at all anymore on Mountain Lion, regardless of whether iPhoto is open or not.
But, I still have Bluetooth devices can wake computer turned off in settings. I'm just so happy to not have the problem anymore, that I don't want to press my luck and cause the problem again.
Have you tried to click "Switch User" from the lock screen? - I mean if your screensaver is on and it's set to ask for password, there is an option to "Switch User", when I click that, my magic trackpad stops tracking, i.e. I cannot move the mouse cursor
I have the Bluetooth devices can wake computer turned off also.