When Loren "Atebits" Brichter released Tweetie 2 for the iPhone last fall, one of the biggest new features in the app was that it was completely rewritten in such a way that the desktop version of Tweetie for Mac and the iPhone version could share the same codebase. From a development standpoint, that means that adding new features to both apps would be easier and more streamlined.
One note about how MacHeist works: Users get licenses for most apps immediately upon purchase, but some apps have an "unlock" number that needs to be reached before serials are given out. Now, this is largely a marketing technique (I've never seen any MacHeist that didn't end with all apps being unlocked for all buyers), but you might not get your Tweetie for Mac license until tomorrow afternoon (or whenever the "magic" number of sales hits).
MacHeist is $19.95 for eight apps (including Tweetie for Mac), plus you can get three other apps for free by taking part in a MacHeist "Tweetblast." The sale runs through tomorrow night and the Tweetie 2 alpha preview is expected to start sometime next month.