This vague O2 page suggests that the tracking facility, which has existed since GSMs inception, may actually be available to end users
However given the lack of concrete information it may be just a pomotion of GPRS, which could be used to transmit GPS coordinates.
On a related note check out this BBC Online article.
Cellphone tracking is really dangerous. You can be tracked anywhere with this technology. It’s scary what can be done using it. Where you go, who you meet, what you buy, what pubs you visit etc can all be found out by somebody following you using this technology. Small cellphone modules can be wired into your car to track you wherever you go. Big brother is here - watch out!!