The poor man's solution here is to just run WoW in windowed mode and have your Twitter browser or client open right next to it -- you don't need a big application if you really need to be connected at all times. But if you want to livetweet your raid without sacrificing screen space to windowed mode, PlayXpert is one way to do it. It'll even do some autotwittering for you if you want.
And of course if you're on Twitter already, don't forget to follow our WoW Insider Twitter account. We've already hit our goal of over 9,000 followers (and yes, a video version of the podcast is coming soon), but we're constantly throwing out little pieces of news, discussion and even some giveaways over there, so follow us if you're interested.