GPS antenna can be seen on many cell towers. Specifically it's used on sites for both iDEN (including Nextel in the US and Telus Mike in Canada) and CDMA (Sprint PCS, Verizon, GTE, Telus, Bell Mobility, etc) technologies. For CDMA, the timing is very important in the broadcast and sequencing of the cellular transmission, so GPS at cell sites are not used for positional information, but rather for the consistant time signal delivered with the positional information.
I run a web site on cell phones that includes pictures of some cell towers, including the GPS antenna. One day I plan to merge the two hobbies and design a multi-geocache cell tower hunt... See http://www.geckobeach.com/cellular/cellpixs/clrn_pix.htm for some GPS pictures on cell sites.
Steve (Licorice)