VMware Fusion 4.1 was released late last week and includes many great improvements. One change was the introduction of a new license verification step for users to verify they are in compliance with the OS licensing terms.
When the license verification step was added in VMware Fusion 4.1 the server edition check was omitted. We are preparing an update.
Running Mac OS X client in a virtual machine continues to require Lion (purchased from the Mac App Store or a USB thumb drive.) Users should always ensure they remain in compliance with any applicable software license agreements.