Bit.ly Pro is the commercial service by bit.ly for pro users and provides more features than the standard service. Users get custom short links, pro dashaboard and a easy setup. Pro users can shortens the links on bit.ly with the custom domain. But if you shorten URLs of websites that are on a pro account, you also get the URL shortened on custom domain.
For example, Mashable uses bit.ly pro and you must have noticed on Mashable’s twitter that all the URLs that they share are of the form http://on.mash.to/b4yHTX. They probably do it from their bit.ly account. But even if open up bit.ly and enter a URL from Mashable for shortening, you’ll get the shortened URL of the form http:// on.mash.to/random. This is true for all websites on bit.ly pro accounts. But for URL bundles, you get bit.ly links in all cases.
Other popular blogs like Techcrunch, ReadwriteWeb etc. also use bit.ly pro when you shorten their links with bit.ly, you’ll get URLs of the form http://tcrn.ch/cPB6eh and http://rww.to/eEXxxz respectively. Foursquare, Yahoo, Pepsi etc. are also on bit.ly pro and the same applies to them as well.