I upgraded my Safari from 5.0 to 5.1 on my laptop yesterday, Safari continual logs me out from gmail.com(hotmail.com,facebook.com) today, even if I set "keep me logged in" configuration. this problem also happened on google, twitter, youtube, etc.
when I send a email by Safari, WebKit2WebProcess.exe stopped working error keep poping up to me. I found that WebKit2WebProcess.exe was created by Safari, so I think it might be a bug of Safari 5.1.
The problem is related with Safari 5.1, many safari users also suffer from this bug after they update Safari from older version to 5.1. These errors affect the Windows version and the OSX version.
We recommend you try to uninstall Safari and install wise pc doctor to remove the damaged files in your windows system, and then install chrome to instead of Safari. CHROME will works perfectly for you.