The new Twitter for Mac application (available from the Mac App atore) has some built in trackpad support (using either a notebook trackpad or the Magic Trackpad). I couldn't find these shortcuts documented anywhere:
3-finger swipe up/down -- This has the same effect as pressing Command+[ or Command+]. That is; it lets you navigate through tweets, mentions, direct messages etc.
3-finger swipe left/right -- This has the same effect as pressing the left or right arrow key. Swiping to the right is context-dependent. It might open a browser page to view link contained in a tweet, or display information about a user or hashtag mentioned in the tweet. Swiping left takes you 'back' to where you were before you swiped right.
[crarko adds: I tested this, and it works as described. One thing about the Mac App store that is a pet peeve for me is that Macintosh programs should be called 'applications' and 'app' should be used for iOS programs. Call me old-fashioned.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110109232851804