If you wish to use AppFabric in high availability mode then you need to ensure that all of your cache servers are running Enterprise edition of Server 2008. This isn't possible for us in our environment.
We currently use a single machine acting as a state server for our web farm. Obviously this leaves us open with a single point of failure.
On investigating AppFabric I found that it was able to handle session state. We are looking at introducing it anyway in our caching layer.
The problem with not running it in HA mode is that if a node goes down you lose the sessions hosted on that box.
I wondered if anyone had implemented a custom state provider which supported AppFabric and asynchronously backed the state off to a SQL Server so that in the event of a server going dark for whatever reason the state could be rebuilt.
I have put no development time into in my idea yet but thought I would float the idea past some clever devs to see if it has been attempted,considered or there are any glaring holes in my idea.
i found here http://netpl.blogspot.fr/2007/06/wrapped-inprocsessionstatestore.html, a solution for having a generic wrapper, but it does not seems quite robust.
Maybe it is possible to switch between providers based upon a particular event or variable.