If you are using Virtual Machine Manager 2007 and Configuration Manager 2007 then you're in luck. Microsoft has a detailed whitepaper available that describes how you can update offline virtual machines. The whitepaper provides guidance and script samples for integrating System Center Configuration Manager 2007 (SCCM) and Virtual Machine Manager 2007 (SCVMM) to provide automated software updates for virtual machines stored in a VMM library (offline). By automating the enumeration and deployment of virtual machines to maintenance host, the IT Administrator can manage virtual machines (VMs) with no additional effort over managing physical machines.

The 77 page whitepaper provides lots of detail and (of course) makes heavy use of Powershell.

Read (or rather download) Automating Software Updates for Offline Virtual Machines Using Virtual Machine Manager 2007 and Configuration Manager 2007 .