Well that's not entrely correct by the facts, though the result will be same.
SUPL (in MSB mode) provides ephemeris and almanac (data about position and trajectory of visible sats etc, needed to calculate your position based on exact sats' position) for the location you're at (+/- a few 100 miles). Usually - without "A"ssited - this data gets downloaded from the GPS sats at a rate of some iirc 50bits per second, taking up to 12:30minutes for one complete set of data to transfer - if you have no reception dropouts during that time.
To get your initial position to request for the right data set, your phone uses different sources like last known position, CountryCode of your GSM/UMTS network (you're 'located' then in center of that country), or even CellID of the GSM cell (tower) you're in. There's however no such thing like "triangulation" for GSM/UMTS. "Normal" SUPL is via internet, no matter if provided by GSM/UMTS or WLAN or even USB-networking/whatever.

What GSM frequently does is: it provides the same AGPS data (Ephem and Alm) via a hidden data channel in the normal network, see the Radio Resource LCS (Location Service) Protocol as specified first in GSM TS 04.31, or http://security.osmocom.org/trac/wiki/RRLP (interesting read! You'll frown at your phone for what it may do on the [hidden] Control Plane Protocol without telling you)

A sidenote: for systematically obscure reasons the N900 SUPL implementation needs a fairly correct time&date on device to work. Even if SUPL needs some time stamp for delivering the right period of Alm/ephem data (instead of a obvious `NOW´), obviously the devels forgot that - when they already have access to internet to get SUPL data from server - they also can get current time by e.g ntp. Not all carriers provide the (correct version of) timestamp on their network that's used by N900 "automatic time adjustment". Probably exactly when they also don't provide CCP AGPS.

/jOERG