UPDATE (April29, 2012): Though WordPress.com now lets you add your own twitter username when using the Tweet Button. For this you need to set up Publicize feature for Twitter. (Official message from WordPress.com here.)
NOTE: When I enabled this feature, it didn’t work at first. So I disabled and enabled again, and it worked fine for a day or two. IT HAS STOPPED WORKING AGAIN NOW.
But don’t worry.
To create your own tweet button, please go through these steps.