Back in 2009 I was doing a post in three parts on the LTE core network and noted that so far I did not look into the details of "Idle State Signaling Reduction (ISR)" which sounded interesting but was optional. Now it looks like Zahid over at the 3G4G wireless blog has done it for me and has produced a great writeup of how the tracking and routing area updates that happen when the UE switches from LTE to UMTS or GPRS can be reduced. Also, have a look here at what Santosh from over at "Wired and Wireless" has written about it some time ago.
ISR is good for both the UE and the network as the former does not need to spend power on location updating and the later has a reduced signaling load. In summary, ISR is activated the first time the UE moves from LTE to UMTS or GPRS and performs a routing area update. If the network is ISR capable, the context in the LTE network with the MME is kept and a new context in the UMTS/GPRS is created. Paging for incoming packets while the UE is in idle is then performed in both radio networks.
One thing I haven't yet figured out from the description is if ISR also has a CS fallback component, i.e. whether an incoming call or SMS is also paged in LTE and GPRS/UMTS. Again, something that remains to be investigated when time allows.