According to this page,
http://www.handspring.com/products/treo/details.jhtml?sub_nav_section=Details&prod_cat_name=Treo
the Treo can use normal GSM1900.

Ok, one more attempt at getting some real info: what signal does the cell mute input line coming out of the empeg cage look for!?

I did some searching, and found these tidbits:

"Some kits such as the Nokia support stereo muting; this is standardized as a ground sensing circuit. To work, a stereo headunit that supports this mutes when a signal wire is grounded; a handsfree carkit that supports this feature will ground a signal wire when a call is connected."
http://www.audiworld.com/tech/elec12.shtml

"We were fortunate in that our choice of aftermarket deck permitted a connection of the Blue/White mute output of the STARMOD1 to the Stereo Mute input wire of the deck. This meant that when the OnStar system activates, the deck automatically mutes. Some aftermarket components require a connection to chassis ground for muting. In this case, simply use a relay or transistor to convert the STARMOD1's +12 volts into a ground for muting."
http://www.carsound.com/columns/navone/nav0010.shtml

I am going to go tear open my dash one more time and ground the cell mute line while the empeg is playing. Hopefully this will mute the output.