I'm a truck driver and I trade with my AT&T and T-Mobile BlackBerrys. AT&T has better coverage, but when they're weak, T-Mobile comes to the rescue.
I'm getting a little dissatisfied with my BlackBerrys, though, because the trackball sometimes get hung at the most crucial times. Since I hate touch screens, I don't want an iPhone.
Verizon Wireless' Motorola Droid got me exited, but the keyboard sucks, so I think I'll pass. (I was able to check it out at a Walmart.)
Regarding brokers, I have accounts at Scottrade, Charles Schwab, TD Ameritrade, and Fidelity - all of whom allow trading with mobile devices.