You can use the Nexus S on any other GSM carrier (SIM Card) provided you unlock the phone... which is quite easy to do on that phone. Granted you will only be able to get EDGE speeds on another GSM carrier. Also, you are better off signing a contract with t-mobile, getting the Nexus S and then paying the $200 ETF. It works out to be cheaper than buying the outright though used ones can be found for $350. You could get this and go with Simple Mobile and get service for about $60/mo with no contract.
The Nexus S 4G can be used on MetroPCS and Boost Mobile, but will require an ESN swap (not illegal if you own both phones no matter what people here might say. Though I'm not a lawyer.) and some advanced settings changed in the case of Boost, and PRL & WAP settings changed (if they add you to their database) at the very least for MetroPCS. Expect to pay about $100 for this to be done to your phone if you don't know how to do it yourself. The Nexus S 4G can be had for $50 on contract with Sprint, plus a $125 credit for porting your number, and then you pay about $70or$80/mo total with taxes/fees for a SERO account. (Look that up). Cancelling a Sprint account will incur a $200 ETF. To get the port in credit you need to stay with them for 3 months so it really isn't worth it unless you want to pay for a few months. Port your number out within the first 30 days and pay the $200 ETF and hope they don't try to charge you the difference for the phone. (It isn't in the contract, but they might try anyway.) Paying the first month then porting out would get around them trying to charge you for the phone.
As something you might not be aware of... AT&T will be getting the Nexus S in a few days. That one would only be able to get EDGE speeds on T-Mobile if you unlocked it. You would however be able to take that phone outside of the country and have it still work most likely. Not sure what the bands will be on it when it is released coming up here though. Also, AT&T has a $350 ETF so it really isn't worth it.
The CDMA phone that is pretty well supported across multiple carriers is the HTC Evo. I would look at that phone as those can be had on craigslist for a mere $200. Still requires you to pay $100 to get it flashed to Boost, but less of a hassle getting the phone.
I know how you feel though. I have a Nexus S 4G and I love it. Best phone I've ever had. The Sprint plan is just way too expensive. I am still within my 30 days so my option is to either return the phone and get about $65 back or pay the $200 ETF and get to keep the phone, but lose my $65 and a $25 deposit and still prolly have to pay the first month service. After all that I'm better off just getting the Triumph on Virgin Mobile and paying $25/mo. (I know it is $35 now, but you can find people to sell you their plan that has already been grandfathered in.)
If you have any more questions just let me know. I know this post has gotten really long, but hopefully this answers your concerns.
