The biggest change in Birdfeed 1.2 is geolocation using the official Twitter API, which embeds the location of a tweet as metadata, unlike previous Twitter app geolocation powers, which were workarounds. It's probably my favorite implementation since it's the dead simplest, with a tap adding location to any tweet. Also new and excellent is support for Flickr, designed in a fairly elegant way. Of course, there's a bunch of smaller tweaks too, and now it's only $3, down from $5. It's still my favorite looking/feeling Twitter app.
Tweetie 2.1 has a bunch of gut changes that add up to a better experience. The big things are that there's now native support for Twitter's new official retweets (love 'em or hate 'em) and Twitter lists. Also, in-app spam reporting (yay) and better geolocation using Twitter's official API. Free update, of course.
Android's most powerful Twitter app is a lot more usable now. The main thing for me is that it finally supports threaded conversations, so you can see what the hell people were replying to. In-app link and photo previews, new keyboard shortcuts, and lots of other smaller UI tweaks. For the future, plug-ins could be huge. The first one is Google Maps, which is nice to have. Oh, and it doesn't look like ass on the Droid anymore.