For the power twitterers:
Seesmic desktop
http://desktop.seesmic.com/ is building an app similar to tweetdeck, that allows you to update both your Twitter and Facebook account statuses and follow your friends on both services.
The difference is that Seesmic allows multiple twitter accounts.
Note that since Seesmic is in pre-release mode, the Seemic twitter and facebook apps are separate, for now. Facebook app here:
http://www.facebook.com/apps/applica...id=23723376453.
For simplicity, you can link your Twitter status to your Facebook status by installing the Twitter application within Facebook. (Just do a search for "Twitter")