My TweetMeMe plugin has been updated and adds support for using alternative URL Shorteners as described in the Tweetmeme help site. You can change this option from Tools > Options > Plug-ins > Options (when Tweetmeme plugin selected). This gives you the following screen:
If you don’t want to use an alternative, you don’t have to do anything, this option is turned off by default. You might also want to use your own API key for a particular service, if this is supported by tweetmeme (see their page linked earlier), then you can put it in using the plug-in (and no, I don’t know why you can’t use your own key for bit.ly).
Anyway, the update can be downloaded from Live Gallery.