The short answer would be this. As long as the phone supports the North American GSM bands (850/1900), and it is unlocked, then it will work with SpeakOut.
Keep in mind that it has to be a GSM phone. Some of the newer phone (like UMTS) phones will not work.
I am looking at the current line up of both Mobilecity and Wind mobile, and they all look like they will work with speakout. Just keep in mind that you might need to unlock the phones for them to work with speak out.
This site is run by a 7-Eleven Speak Out Wireless customer. I am not a 7-Eleven employee, nor do I receive any compensation from them for providing this site. The goal is simply to share consumer information with others.