Hello Pix,
I made some research on the topic and found out that this feature can be implemented on pre-release 8 mobile phones but an update has to take place first on the handsets.
About fast dormancy feature, it is used to control the excess smartphone signaling in the network. Smartphones usually moves from DCH state to Idle state directly in order to save the battery while it should pass by FACH state for example. The new feature makes use of the PCH state and introduces a timer T323 that makes the smartphone unable to release the connection before a certain time.
U can read more on this in TS 25.331, chapter 8.1.14