Microsoft Dynamics CRM 4.0 is compatible with Windows Server 2008 based computers, however there are some known issues at this stage.
We have published a Knowledge Base article that detail all the issues, including any workarounds that might be available.
This article can be accessed here or by searching for KB950100 on our support pages, and will be updated frequently with any new information.
It is also worth mentioning that Microsoft Dynamics CRM 3.0 can be installed in a Windows 2008 domain, however the computer where you install Microsoft Dynamics CRM 3.0 must be running Windows Server 2003 or Windows Server 2000.
Microsoft Dynamics CRM Technical Lead.
EMEA Customer Support & Services - SMS&P