Found some info
Straight Talk: It could let you dump AT&T or T-Mobile — Mobile Technology News
Review: The iPhone on Straight Talk | 9to5Mac
Currently, it is technically against the terms of service to use any other phone than the ones they specifically sell, though they haven't been checking the IMEIs unless you called for support. However, you would need to purchase a $50 Nokia phone (refurbished), activate everything on that device, and THEN transfer the SIM to the iPhone.
Here's a good guide on getting it set up:
http://wiki.howardforums.com/index.p...ht_Talk_iPhone
The Nokia devices come with SIMs that use AT&T's network.
Starting on Feb 7th, Straight Talk will be selling SIM cards for unlocked devices (though you wouldn't necessary need unlocked if you have an AT&T iPhone anyway) as part of a new service... only initial cost will be a $15 SIM fee (much better than the $50 phone), AND you don't have to worry about them terminating your service by using a non-official phone, since this new service is BYOD ("bring your own device"). Still $45 unlimited everything, and perfectly legal now. (Note that you cannot use a CDMA iPhone 4 or 4S with Straight Talk. It must be GSM)
Here's the site for more info on the SIM offer, starting Tuesday:
Welcome | Straight Talk
One concern, that will still hold for the new BYOD service is the data limitations. While it is technically unlimited, they have cut some people off who use high amounts of data (i.e. 5-10+ GB per month). It is recommended by people online that you try to stay below 2GB per month if possible. However, people who have gone over have been able to call up (they claim to have said they left YouTube open), and have their data re-activated. In general, it seems as long as you don't go crazy with data usage, you'll be fine.
Some other things to note include the 3G speed, which is usually around 2-3Mbps down on the 3GS and 4, and I've seen screenshots of up to 5-6Mbps on the iPhone 4S. The carrier in the top left of the screen (next to your bars) will read "HOME" rather than "AT&T". You can change this if you'd like if you are jailbroken by using an app called FakeCarrier.
3G data, Push email, FaceTime, and iMessage all work if you use the APN of "att.mvno". MMS works for some, though may require a jailbreak application. I'm not concerned about MMS though, and I don't jailbreak. I rarely receive them, and I can send images as emails if I need to. Personal hotspot also only works for some, but there are some ideas out there on how to activate it without a jailbreak, though it is technically against their Terms of Service, and you could be red-flagged in their system if you use tons of data tethering all day.
I hope this post has helped; I will most likely be ordering the new SIM next week to use this service on my 4S, finally an upgrade from EDGE on T-Mobile, which I use currently.