The co-founders of Twitter are currently discussing ways to integrate video tweets in their service. This would enable people with video capable mobile phones to upload clips to their profiles. However this would consume a large amount of bandwidth which Twitter is struggling to deal with already.
There are are a couple of third party video websites that have this type of feature already such as Twideo and Tweetube. But these two video websites have not gained as much traction as the Twitter photo-sharing third party website TwitPic.