Drag and drop into your bookmarks bar. Only tested in FireFox -- sorry.
(Works best if you simply doubleclick on the username to select it. Don't include the "@" symbol!)
If you like it, leave a comment! If you hate it, go to Hell. ;}P>
This was made for @divabat. My twitter is @drjon. There's two you can try!
ED: I have also hacked a couple of Twitter Search bookmarklets, which might be useful.
twiTag (Twitter Hashtag Search)
Again, drag and drop into your bookmarks bar.