Twitpress is no longer in development. If it’s not what you need, take a look at some of the projects mentioned in the comments below, TwitCategory in particular looks good.
Twitpress is a really simple WordPress plugin that does one thing: Every time you publish a post on your blog, it tweets an update to your twitter account. This lets you notify all five of your twitter followers that you’ve posted a new blog entry immediately! You can format the message and optionally include a link to the blog posting to minimise the navigational pain for your devoted fans.
Twitpress can be downloaded from the wordpress codex.