Just found a rather nice App for my phone. I don't know if this is also for iphone. I have an HTC and it works a treat. You need a wifi connection at home though.
It is called FSMovMap. It is free and can be downloaded from "Market". It links your flight to your phone as an external moving map display. No need to lose screen space on your computer or shrink down the GPS screen into the corner. Just follow the instructions. It worked out of the box so to say for me. You can either have the standard map display or Google satellite map. I have not noticed any FPS reduction. As far as I can see there is no effect on FSX at all.
The only extra thing you need is a server program from rahsim.com This is the home page of FSMovMap


Have fun

Matt

Hmm....does it kill battery life? That's all I care about with my android......