Why am I so excited for them? Mostly because they're extendable! (Well...and I guess because I wrote most of the IRC code.) I've written a bit about this before for IRC...but it will let add-ons do whatever they want to the IRC protocol. You should check out the implementations (links above), they're very hackable. Hopefully we can remove libpurple XMPP and fully switch to Instantbird's XMPP for the next release, once a few Mozilla bugs are fixed.
Did I also mention that these implementations (including the raw XMPP and Twitter, which Instantbird has supported since 1.0) are going to be included in Thunderbird 15, as part of it's new chat feature? Florian has done a great job of integrating our chat code there and it gives quite a different user experience than Instantbird, so don't be worried about Instantbird going away!
Now of course, we always think of the future here (after all, releasing itself isn't really exciting when most of the features have been in nightly builds...forever), so we started making a list of some of the stuff we'd like to implement in future Instantbirds, you can check it out here. Some of them are very exciting, feel free to grab one and work on it.