I have a mobile web site and I needed to do IP geo-location. I had a look at several IP->Location databases. One of which was MaxMind. They have a free database which provides city level accuracy, but they also have a paid for database (for a pretty reasonable fee) that gives you more detail, including what mobile carrier a mobile user is coming in on.
Go to http://www.maxmind.com and put your IP into the demo entry box (its on front page), and you'll see the detail you can get. This is the API you would need from them: http://www.maxmind.com/app/isp
Like I say, I only use the city level detail database so can't vouch for the coverage of the mobile carriers. But during my initial tests, it did always seem to return good values for the UK, Ireland, and Asian mobiles I tested.
Rgds, Kevin.