I'm not an expert at linking old threads, but this has been discussed previously if you search.

Credit cards issued by Juniper Bank will give you miles for balance transfers. These include Midwest Airlines and Frontier Airlines (possibly others). I know that Midwest Airlines currently will give 1 mile for each $ transferred up to 10,000 miles. However, there is a $50 BT fee for this service. Midwest Airline miles can be used for awards on American Airlines and can also be tranferred directly to Hilton HHonors.

Others more knowledgeable than I can give you specific input on BT's using United Airlines' credit card. I know that they have historically offered miles for BT's, but I don't know if this is on-going.

EDIT: Gary Steiger has a good website that describes the various credit cards that earn miles. I believe he also describes which ones earn miles for BT's.