Straight Talk now allows you to purchase a SIM card without having to purchase a telephone from them. The link is on the Straight Talk homepage. Welcome To Straight Talk
Click on the red "Get SIM" link on that web page.

Looks like you activate a Straight talk SIM on another phone,
then pop it in the Atrix and change the APN.
See here>>>Motorola Atrix 4g on Straight Talk Sim Card - YouTube

did you have any luck finding out about using ST on an atrix 2?. I would like to do the same. Thanks

I'm currently with AT&T and have a Atrix 4G. However I'm pretty disgusted with AT&T right now and am seriously considering paying the early termination fee and going with Straight Talk. However ST's selection of smart phones is sad at best. I was wondering if I had my own non branded Atrix 2 or even if when I canceled with AT&T if they unlocked my 4G would either of them then work with Straight Talk?.

I looked into Virgin Mobile already and from my understanding they use the same towers in my area as Sprint and Verizon and in my particular area Verizon is practically un usable from weak or non existent signal while with AT&T it's rare to have less than 4 bars even in the house. In our area at least Straight Talk uses AT&T towers.

Another big drawback with Virgin is they don't offer a world phone and we usually spend 3-4 weeks a year in the UK visiting my wife's family. With an unlocked Atrix I could simply get an 02 or Orange sim card and not get raped by AT&T with their international roaming charges. I unlocked my Atrix 4G using a third party service before we left for england last month and while a o2 sim card would get me voice the o2 tech couldn't get me data or even wifi. Not sure why exactly unless I kept getting o2's bench warmers of tech support.