I worked in a walmart wireless store for a few years during college, and like cgardner said the straight talk branded android phone will probably run on sprints network. During the time I worked there I had to process more returns then I can count of straight talk android phone because they run off sprint towers and where I live sprint has very poor coverage. Some straight talk phones use ATT towers, some use verizon, and some use sprint. Not sure if this has changed but when I worked there you could tell which towers each phone used by looking at the box, and it was written in one of the corners can't remember which but look for these words. If it says "GSM" then it's probably AT&T (Don't think any use tmobile), "CDMA" means verizon, and "CDMA-S" means sprint.
As for using a galaxy s II you get from AT&T on straight talk I'm not sure. I would think since it has a sim lock it wouldn't accept a straight talk sim, but I could be wrong.