SL does not detect display on launch

I use two monitors: an Apple Cinema display and a Dell, which is display number 2. I installed SL and it was normal until I rebooted the first time. Now every time I boot up, the only display that is found is the Dell and I have to go into preferences and select "detect displays" to make the main one show up. It loads as if the Dell is the only display, and since it is smaller it messes up my desktop, lumping all my icons on top of each other. When I click "detect displays," it moves everything back to the Apple monitor, making it the main display as it should be, but it is all out of whack.

Any thoughts on what my be the problem? Thanks!

 

Thank you for the suggestion. I will try that tonight. I appreciate it!

 

I finally tracked it down. I had a my desktop set to change the wallpaper every day, and when this option is enabled it doesn't find the main screen. If I disable the rotate wallpaper function, everything seems to work as normal.