What brand of phone? What kind of I/O are you trying to use? Can you say what you app is supposed to do?
I've written a couple of simple programs that run fine on a phone with Windows Mobile 5.0. But they were very basic, with some graphics, tab controls, ring selections and simple calculations. My main usage for Windows Mobile has been communicating with a relay board, and since the board uses a serial port, most smart phones are not useful for that purpose.
I'd say the only way to really find out is to try it.
Remember though, NI requires a licensing fee for PDA apps, so if you are going to sell or distribute your programs, you will have to pay for it.
B-)