Not even close to that -- more like 50-100 meters.

True, but A-GPS is being used in CDMA networks.

Much more reliable, but much less accurate -- on the order of 100-300 meters.

In practice it's not that easy.

Matt, it's far more comprehensive than that.  In a True Position U-TDOA
Phase-2 production network, every street, alley, highway, and parking
lot is field driven with a GPS equipped vehicle, overlayed onto a street
address map, along with a repetitive-call simulator feeding raw data
into a resultant look-up database of known GPS coordinates tied to
timing specs from as many BTS sites (>3 equipped with LMUs) as could
track the simu-calls.  This process creates a GPS-calibrated timing map
overlayed with street addresses.  When a handset initiates a 911 call,
multiple timing specs previously recorded from that location match a
GPS-calibrated street address, which is then provided to the PSAP before
a voice channel is assigned (~100-300 msec).

So, while the handset doesn't have it's own GPS receiver, the cellular
system was calibrated with GPS - and signal timing specs, when repeated,
provide a level of location accuracy well within FCC requirements.  It
works with any handset capable of using the system, is unaffected by
weather, overhead obstructions, and signal reflections have already been
accounted for.  I've heard on rare occasions, a handset's timing signal
from one BTS will anomalize, maybe due to recent construction of nearby
objects creating a new (unknown) reflection - and is tossed aside - but
other BTS timing details are adequate to afford a successful placement
of the handset's location.  I've also heard these anomalies are tracked
for trending, and subsequent re-calibration of these areas is regarded
as a standard maintenance issue.

I think if one would peruse the True Posotion website, a number of
carriers are listed as clients, and a more comprehensive explanation is
available.