Past tense. Garmin sold a AMPS phone built into a GPS, but it is kind
of ancient history.  800 Mhz Amps coexistence is probably a lot easier
than PCS/GPS coexistence. The bind is that all filters have losses,
and on the mobile phone side, you can keep the signal levels high
enough to live with the duplexer losses.  The margins in GPS aren't
all that great to begin with, which is why the data rates are so low
(about 100 bps).  

The main use would be for 911/999/000 type services, and there are
other methods available that work better in built up envirornments and
inside bulidings.  The basic timing data from a GPS BTS can put the
phone inside a cirlce that is about 1 km in diameter.  With a second
BTS, and careful measurement of the timing at both, you can get the
CEP down to about 10 meters.

In short, not a good reason to do it, and the Garmin product was a
flop..

You will be surprised. Have a look here

http://www.gps-practice-and-fun.com/gps-phones.html

regards