Just ran into something interesting - not sure if this is a Known Issue or not.  Using Safari 5.0.5 on OSX 10.6.8, I was authenticated into a Google property (gmail). Switched to "Private Mode" in Safari, then attempted to log out of gmail and switch to a different account. Even after clicking on "Sign in as a different user" I am still returned to the same page, prompting me to authenticate the last user logged in.

Upon exiting Private Browsing, the user is still set, but now upon clicking "Sign in as a different user" the expected functionality happens, and I'm able to enter a username.

I'll admit this is a hurried post on a busy day - this may be known behavior with Safari Private Browsing, but seems like something is falling short - there's a small chance for information disclosure, here.