Hi to everyone,
I see on the Apple Support Communities also other users have my same problem: I have a iMac on which are enabled both Screen Sharing and Fast User Switching and it stays switched on all the day with a logged account but in the lock screen (because after 5 minutes of inactivities the screen is locked automatically and ask again for the user password).
When I connect remotely via Screen Sharing from my MacBookPro (or iPad or another computer) to the iMac, the iMac freezes (it appears the coloured circle) and the only way is to force the swhich off of the iMac by pressing and holding down the power button.
I also see that if I connect to the iMac from the MacBookPro (or iPad or abother computer) while the user is logged in and it's not in the lock screen, there isn't the problem; so the problem only occours when I try to connect to a Mac via Screen Sharing, with Fast User Switching enabled and with a user logged in in lock screen.
I also tried to format another mac (of my friend), enable the Screen Sharing, enable the Fast User Switching, login with a newly user, lock the screen and connect with another mac (o iPad) and the problem also occours.. so I verified that the problem is not in my computer configuration, but is in Lion.
I contacted the Apple Italian Support (using the 30 days support of my friend's just bought computer) and reported the problem; none of the solution they've provided to me solved the problem, so the Apple Italian Support recognized that it's a Lion bug and told me that it will be reported to Apple's Development Team.
So if any other user has my same problem, he could reply this post and contact it's local Support Center reporting it, so that more users report this bug and more possibility we have to see it solved.
I hope this post could help someone.
Have a nice days.