You may experience issues opening documents direclty from libraries using Office 2010, here is a work around to the error below:
Solution:
set up a fake proxy (127.0.0.1) in Internet Explorer with an exception for *.*. Still don't know why this works, but it has allowed me to actually check out, edit, and upload/check-in documents from our libraries.
Credit:
http://social.technet.microsoft.com/Forums/en-US/office2010general/thread/700bdff2-229e-4e90-8aae-d0c90c54a3d1