The same thing happened to me inexplicably.
It sounds like everyone is getting the gray textured background during login (it looks like a gray woven tack-board to me) which they find to be undesierable. I don't like it either, but mostly because I thought it may have been booting into some Safe or Recovery mode which I believe have this background; whereas before the login screen was different.
I believe it has something to do with FileVault. Shortly after getting my MacBook Air and setting up for the first use, I turned FileVault ON to encrypt the local HD. Ever since I first logged onto my computer the start-up sequence has been the following:
Power on -->Bright white screen --> The Apple logo on top of the circular user name icons which are on top of Restart and Shut Down buttons, slowly fades into view --> after choosing a user account and entering password, everything fades out except for the Apple logo that drops down to the center of the screen --> the activity icon makes a couple of revolutions and the Desktop slowly slides into view.
Now, a few days ago, I attached the USB Superdrive to install Diablo 3. I was unable to install the game for some reason (what a surprise) and, after searching troubleshooting guides, I found that the game was incompatible with FileVault. So, I turn FileVault OFF, it has to resart the computer before decrypting the HD. It was after the restart that I first noticed a different login screen and sequence from then on, which goes like this:
Power on --> large Apple logo appears and the activity icon makes several revolutions --> then the gray textured background appears with only the round user icons and three buttons below them (Sleep, Restart, and Shut Down) --> after entering password, an activity icon appears within the password field, makes half a revolution before the Desktop slides into view. Startup was a bit slower as well, I noticed. Also, my screen saver was reset to the default (if there is one).
Anyway, after much searching, I decided to try turning FileVault back ON and after the restart, the first login sequence I described returned without the gray textured background just like it always was. Kind of wierd, don't know what is going on behind the scenes but seems to have something to do with FileVault.