This is an odd one - an admin had created a share and sent it to a few users. All was fine. I received the invitation for the share but there was no option to accept the share. We found that the only way to see the option to accept the share was to switch from ajax/advanced to standard view, click on the message again - then everything worked fine.
This only affected my account and we tested my account in two different locations - different pc's. 1 running windows xp home w/latest updates (just last night), problem occured in firefox 2.x and I.E. 7.x