Depending on your area, Straight Talk may use AT&T or Tmobile.
If you are in a hot AT&T area like I am, you're set.
There are a lot of considerations to know & understand.
- I wouldn't invest much in a 3GS ($50 max?)
- a handset locked to AT&T is fine on (AT&T) Straight Talk 'cause it IS on AT&T

- getting it set up with talk/text/email/Internet is not hard; you just need to follow a proceedure
- getting MMS (picture messaging) working is the challenge (I used a technique that relied on jailbreak)
- you can request free sim-unlock from AT&T, or I have a cheap 3rd party under $5 (may not need either)
- put a screen protector & case on it: $1 & $3 @ Amazon!
- be aware that users can easily upgrade firmware with iTunes , & probably shouldn't
- I can recommend car & home charger $4 @ Amazon, if you need
- anybody can set up their own iTunes account; even without credit card (to get free stuff only)
I spent the last year repairing 3GS & several iPhone 4 handsets.
... and shopping Amazon for parts & accessories ( and testing many).
So happy to recommend; have many bookmarked.
