Those of you waiting on the Official Twitter app to get a Facebook-like major update will be dissapointed to know that's not happening. A new, minor version was just pushed to the Marketplace and while it is now Mango-ready (meaning fast-app switching) there are no new features on board--no Live Tile, push notifications or anything significant we've noticed. The version number was minor too going from 1.13944.31667 to version 1.14329.14900--so try to calm you excitement.
It is relatively fast, supports multiple accounts and looks decent enough. So for those with basic Twitter needs, it should be all right. You can grab the official Twitter app here in the Marketplace.
Update: So there are a few more updates in here. For one, you can use Twitter's pic service for your photos (thanks, Amir). Two, it's in the Russian Marketplace now (thanks, @achkasov) and three, it has Arabic language support (thanks, @HmoudAdel). So that ain't half bad, we suppose.