I just wanted to make sure you all keep in mind that the majority of cell phones which market "GPS" capability in fact do not use GPS (global positioning system) at all!
Nearly any cell phone can use cell phone tower triangulation to give a pretty descent approximation of where you are - my Palm Centro (great smartphone btw) using the Google Maps "My Location" functionality gives me position to about 1000 ft. However, this functionality does not work at all if you are not on land! This is because your cell phone radio is simply not strong enough to transmit to 3 different cell phone towers simultaneously when off shore. Also, the typical position accuracy of a real GPS is 10' or less - 100X better than cell phone triangulation.
That said, if you DO get a phone that has real GPS capability (it recieves signals from 6-10 sattelites orbiting the earth), you need to investigate if it can transmit the position information (via USB perhaps) to other devices (i.e. a PC). There are definitely plotting tools you can use if you can get the position info to a PC.... but realistically, the integrated plotting features of marine GPS units (i.e. Garmin) is probably worth it!
S/Y Carpe Diem