Unfortunately the Mail app only works over a SSL connection, which apparently Speakout isnt. What you can do is use Safari to access your inbox through Yahoo. You'll have to initially log in on your wifi network, after that you should be able to log in on 2G. In Safari, while on your inbox, press the + button, that "Add link to home page". This will put a nice yahoo mail button on your springboard that will take you directly to your mail account when pressed. One last thing, download an app called UAFaker from Cydia and activate it. This will trick Yahoo into thinking you are on wifi and will allow you to read your mail, forward, respond, etc.

Could be any number of reasons. More than likely they just don't have a problem with 3GS phones on the 2G network though. Why would you spend the extra cash for a fast phone just to use it on a 2G network. Or perhaps Rogers tied to block all iPhones and did a half ass job of it. Whatever the case, nobody is going to be able to give you anything more than speculation on this.

I've seen people attempt with all models of iPhones, and I have not seen anyone succeed with the 3G.

Really can't help much, but i'd try looking on cydia for a toggle that would disable 3g. It's possiible that the built in switch is not completely shutting it down. I don't have a 3g to experiment with, but would love to try. Unfortunately though I've still not seen anybody that has had any luck with a 3g.

I don't see any reason why it wouldn't. My guess is that the 3G was blocked via it IMEI number when Rogers was actually using the equipment they now lease to Speakout. Unless they decide to start filtering out other models the 4G should work fine.