.NET Framework 2.0 Update for Exchange 2007
Microsoft released a .NET Framework 2.0 CLR update that should be applied to every Exchange 2007 server. It is more urgent to install this update on Client Access servers with multiple protocols, but Microsoft recommends installing the update on all Exchange 2007 servers.
This fix will be included in .NET Framework 2.0 SP1, which doesn't have an ETA for its release.
More information about this update can be found in the Microsoft Knowledge Base:
Source: Scott Schnoll's Blog