Pete Cashmore has a blog post that covers this well with a screenshot so I won't repeat the details here. 

This brings up the question, where does the line get drawn with others copying? Being copied is supposed to be the ultimate form of flattery but.. facebook put twitter-like functionality, now bebo, next My Space, Flickr and everyone else (guessing here!). The good thing for Twitter is that they have momentum and a community starting around their service. They also have the brand name so I think it would be difficult for a new standalone Twitter to be successful. A...