More Alphabet Soup

IMS - IP Multimedia Subsystem
CSCF - Call Session Control Function
Various types including S (serving), P (proxy), etc
Think SIP Proxies
P-CSCF is the first point of contact for a user in IMS
HSS - Home Subscriber Server 

GPRS - General Packet Radio Services (aka 2.5G)
{Serving,Gateway} GPRS Signaling Node (SGSN/GGSN)
In IMS, the P-CSCF is always in the same network as the GGSN (this will become important when thinking about roaming, trust me...:-)

2G and 3G Radio
GSM - Global System for Mobile Communications (2.XG)
UMTS - Universal Mobile Telecommunications System (3G)
UTRAN - UTMS Terrestrial Radio Access Network