Smartphone Decision for Young Programmer?

Switching to T-mobile soon and want to take advantage of their excellent data plan, I also have need of a "rough" GPS device although a fully functioning one is a plus. I know my way around enough that just having google maps 'should' be enough.

Anyway, my choices are any smartphone that is unlocked and GSM. My primary contenders are

Palm Treo, for the price factor since I can get one for under $200 unlocked.

Apple iPhone, for under $500, although the totalitarianism is a serious turn-off.

Any Good Windows Mobile Phone?

Nokia N95, heard it was good, but the reports of cheap construction and laggy interface are a huge turn-off for me. I hate laggy interfaces. Love the GPS though.

Who among you are very happy with their smartphones?

Ability to do wifi is important, GPS is a plus, a good ecosystem of applications is a near-must since I'd like to tether without paying the ridiculous fees for it.

Hackability/Open Platform is a plus, I'm a programmer so something I can tinker and learn on, and provide my own solutions to small problems is a big boon for me.

A decent web browser is a small plus, but Opera Mini is near ubiquitous, so it shouldn't be a huge deal.

Please offer your suggestions for phones!