The Kinect update from Xbox Live on November 1, 2010 caused a large number of Xbox 360 to stop reading official discs and only give an "open tray" message on even non-modded consoles. Microsoft still hasn't fixed the issue. There is also a new January 19, 2011 update that is now also giving people similar problems.
There is a very large topic on the November 2010 update problems at the official forums. But it was recently closed by the moderators...
http://forums.xbox.com/34833009/ShowPost.aspx
There are some new topics on the January 2011 update problems. Simply sign into the forum with your Microsoft Live ID and add your complaint to it to pressure Microsoft to fix it...
http://forums.xbox.com/9/ShowForum.aspx