Vista - 3GB VM Limit Under Vista x64

I've been running through some trouble-shooting tests to figure this one out,
and the conclusion I've come to is that 64-bit Virtual Server 2005 R2 SP1 (or
64-bit Virtual PC 2007 SP1) under Vista Business x64 will not run more VM's
than can fit in approximately 3GB of memory, regardless of the memory
installed in the host machine.

Running three 1GB VM's with Server 2008 installed is fine, but when
attempting to start a 4th, Virtual PC reports "not enough memory available on
host", whereas Virtual Server reports "not enough physical memory or system
resources available".

What I'd like to know if this is a known issue with Vista, and if there is a
hotfix available.

I've run into a problem when attempting to start a fourth VM configured with
1GB memory.

64-bit Virtual Server 2005 R2 SP1 reports "there is not enough physical
memory or system resources available".

Host OS is Vista Business x64 running on Core 2 Duo vPro with 8GB main memory.

I've changed the memory configuration of the VM's to 512MB, and I can run
six, but the same error is reported if I try and run a seventh.

Is this a known limit with Vista?