I've not attempted to get speech server running on Server 2008 as of yet (it is on my list however) so I can't give you specific details on getting it working. That said, I don't know of any reason why it would fail. It should be said that this is an unsupported configuration by Microsoft. That could be because they've deemed it unnecessary to qualify it or it could be because they know about one or more issues that Server 2008 introduces.
Virtualizing your speech platform in any way is not a good idea. Speech processing requires high CPU, disk and memory utilization which makes in unattractive for virtualization. The idea behind virtualizing your resources is that it allows you to share resources across applications, the assumption being that no single application is using 100% of the resources at any given time. With speech telephony this is simply not the case.
- Marc LaFleur
http://gotspeech.net/blogs/speakingfromtheedge/