I called T-mobile first, and asked them about transferring my number to another provider but keeping my T-mobile account and getting a new number assigned to it. They said sure, to call them after I switched the number over.
I called them a few minutes after the line was set up with Sprint. They waived my early termination fee and re-activation fees. It took them about a day to be able to re-use my existing sim card, but then it was working with the new number.
A few days later, I made a deal with someone on one of those phone transfer websites. I gave her a $40 bonus and mailed her a phone (a cheap old Samsung) once I confirmed she had taken over the contract. I will sell off my old sim-card-based Treo, and the sim-based rotary cell phone. Anyone interested in either for around $100? :) They'll go for more than that on Ebay. No? How about $80? :) They'll work on T-mobile or AT&T or (I think) any sim-based network.
It was a lot of hassle, but a lot cheaper than either early termination fees or paying for the extra account through June.