iPhone 3G is not unlockable and hence you won't be able to use it with T-Mobile.
Also iPhone 3G uses the UMTS (3G) GSM 850/1900/2100 Mhz bands to connect. T-Mobile in the US is UMTS 1700Mhz. Not compatible... you will only get EDGE network.
Using a TURBO-SIM like approach for the 3G is considered very illegal in most countries since the methodology it uses to connect to the cell networks is very illegal itself. The iPhone Dev Team has recommended not to use that method.
Yes it is and i have a program that will unlock the 3G...i just dont have one but it unlocks both 2G and 3G.
I see unlocked 3G's all over the internet for sale(ebay, craigslist)