I use IB and Fidelity apps on my iPhone (which has 3G) and iPad (which stays at home) and they are fine, with limitations.
If you're on-the-go, there will be places (like tunnels) that won't have a signal.
On an iPhone and iPad, you'll be limited to running one program at a time, so if your broker's app doesn't provide everything you need to trade (such as news stories), you'll be spending time switching between apps.
Alerts are another thing to consider. Some trading apps will only pop up alerts if they're the currently running app, while some will alert you even if they're running in the background.
Bring along a spare power source for when your battery runs out.
These problems largely go away if you carry multiple devices, but that can be unwieldy (like, when you're standing on the bus).