When I went to the main page this morning, the "Welcome to our site" banner (or whatever it says) obscured the top panel containing the "log in" button.
This was on Firefox 3.6 (technically speaking, Namoroka) on Linux.
This is by design -- if the greeting is 2 lines wide (depends on length of message being presented), it has to be larger.
Just click the [x] to close it and proceed.
I'd noticed that too: I wonder if it is a bug of a 'feature'!