Bit.ly exploit to brand your short url

I don't know if someone has observed this, but if I append any random sub domain name, to a shortened url from bit.ly, it still takes me to the correct url.

For example, I picked up this from techrunch's tweet http://bit.ly/12V9c1. I could also create a link called http://mayanks.bit.ly/12V9c1 and it would still take me to techcrunch's site. So I get to brand my own short url.

Is this an undocumented feature or something is wrong here?