Oh yeah, I tried that before and it doesn't save that setting. It never switches back to 2G only and once you back out of Field Test and get back into it, its set to Automatic again.
It does keep it on 2G for me. It shows "automatic" when you go back into Field Test, but unless you actually change it, it doesn't modify that setting. Test it out. It will never go back to 3G if you set it as 2G only. I'm not sure if it resets to Auto on a soft-reset, but how many times have you actually had to reset your device?