Adobe Flash Media Server 3.5 (FMS 3.5) is now available to pre-order. It will incorporate new features including Dynamic Streaming, DVR functionality, HTTP delivery support, and H.264 enhancements.
The upgrade price in the US is the US$349 (depending on your existing edition), with UK pricing being in line with that (and by that I mean: just change your currency symbol since the UK price puts the exchange rate to the US dollar more or less at 1:1 with a price of GBP 346.62).
You can download the current version (FMS3) from the Adobe website and give it a try.

Apologies for the earlier misinformation, I was under the impression that FMS 3.5 had actually shipped.