This article discusses a Congressman using Twitter and how it may fit within House rules.

Texas Rep. John Culberson uses his Blackberry to post blurbs about his work onto Twitter, a social networking site on the Internet. The Internet has set him free from unfair media reports and other barriers between him and his constituents, enabling him to better represent them in Congress, he says.

But Culberson’s actions have put him in possible violation of House rules that appear to ban blogging or other work-related activities on non-House Web sites.

Another site asks,  “Is Congress afraid of the internet?”  This post has more context.