Several Mac users are reporting that Mac OS 10.6 (as well as 10.6.1) will spontaneously log out of their account in the middle of using applications. The issue does not appear to be related to specific applications running as users have reported it happening while using Excel, Word, Safari, Photoshop, and Filemaker Pro among others. Most guesses have the problem related to Rosetta running on the new operating system, though an official fix has yet to be reported.
Apple Support Discussions forum poster "Stu Baker" writes:
I am having spontaneous logouts while I am in Snow Leopard. This is happening a couple times a day and it is very annoying. When it logs out it instantly goes to a blue screen and then the login screen appears. When I log back in all my apps have quit and it is like I am logging in for the first time. I did have this happen in Leopard too, but very rarely.
I attempted to recreate the issue on our Black MacBook running 10.6.1 with Rosetta installed. Since updating to Snow Leopard weeks ago and daily running Entourage 2004, I have yet to experience a spontaneous logout. One forum poster suggested a complete erase and install of Snow Leopard. If you are going to attempt this, be sure to have a backup of all your important data. For now, it may be worth considering the applications you need to run before upgrading to Snow Leopard. If you have a heavy dose of programs that will require using Rosetta in Snow Leopard, you may think about waiting until this issue is officially resolved.