So what is this IMS thing?

The IP Multimedia Subsystem, or IMS, is being positioned by the 3rd Generation Partnership Project as THE architecture of choice to converge: 
Wireless and Wireline networks
Voice and Multimedia data

Basically: IMS is an attempt to create a telephony standard infrastructure for IP services that will (in theory) evolve into a replacement for circuit switched voice. 
IMS is also being proposed as the wireline services control plane. Result: You might see IMS (or some subset of it) sooner than you might have otherwise expected