Account, Contact and Lead now include a Jigsaw field, which will reference the ID of a company in Jigsaw.
FeedPost will be deprecated and FeedItem should be used instead.
Also, FeedPostId won't be on record feed items, and the corresponding fields on the record feed item, like Body or ContentData should be used instead. For a full list of effected or unavailable objects, see the full release notes.
For Profile, the permissions fields now support filters within SOQL statements.
Of course, these points don't cover major changes like the Chatter API Developer Preview. And if you want to try out that Apex REST API so that you can roll your own REST framework – just give us a ping.
For more information on all the goodness coming down the pipe with Summer '11, check out the Summer '11 Release Page, which goes into all the new major features, and provides link to the release notes PDF as well as the recent webinar.