From Business Communications Review: IMS 101: What You Need To Know Now.
"Out of the wireless standards consortium called 3rd Generation Partnership Project (3GPP) comes a slow-growing and complicated collection of carrier network functions and processes that collectively are referred to as IMS, which stands for the IP (or Internet) Multimedia Subsystem. The IMS standards promise an operator-friendly environment for real-time, packet-based calls and services that not only will preserve traditional carrier controls over user signaling and usage-based billing, but also will generate new revenue via deep packet inspection of protocols, URI and content. IMS was conceived for the evolution of cellular telephony networks, but the benefits of user signaling and billing controls have attracted the endorsement and involvement of wireline network operators and standards makers, including the European Telecommunications Standards Institute (ETSI), the U.S.-based Alliance for Telecommunications Industry Solutions (ATIS) and the UN-sponsored International Telecommunication Union (ITU). In the U.S., cable multiple systems operators (MSOs) are also showing interest in IMS as part of the recent CableLabs PacketCable initiative, and network operators recently approached the WiMAX Forum's Network Working Group, asking that IMS be included in its forthcoming reference architecture."
From Ewan Sutherlands weblog.