Mine is doing the same thing now that it's getting warm again (mid 90s earlier this week). In the morning or late evenings, the screen looks perfectly fine.
It's a common problem and people have had the head unit replaced under warranty because of it. If you're out of warranty like me, not much you can do besides replace it or have it repaired. There are people/companies who repair them but I haven't done much research on that yet.
Here's a DIY for replacing the LCD screen if you're feeling up to it
http://www.e90post.com/forums/showthread.php?t=635672
I've decided to live with it until the screen becomes completely unreadable.
Edit: Here's an example of someone to repair it albeit they're in the UK. I'm sure there's someone in the US that does it too.
http://www.diyparadise.net/bmw_cd73_repair.html
Edit 2: Here's one in the US
http://www.bimmeronly.com/BMW-CD73-C...ies_p_221.html