Absolutely (as long as it's x64, not ia64). This topic came up at work, and there seemed to be a fair bit of confusion over what's possible and what's not. Using VMWare Server (or a Workstation version that supports 64bit guests) you can run a x64 Guest OS even though the host OS is still 32 bit. Unfortunately, neither Virtual PC nor Virtual Server 2005 R2 supports 64bit guests.

That means you can run Windows Server 2003 x64 with Exchange 2007 (which is 64bit only, except the trial edition) on Windows XP/Vista 32bit. If you're consultant like me, that spends a lot of time in other people's offices, it means you can use a laptop with a Core 2 Duo CPU (provided it also supports Intel VT - i.e. not T5200/T550) and run 64bit guest machines.

VMWare has a free tool you can download to check whether your machine can run 64bit guests.