This SearchServerVirtualization article talks about how Xen is coming to market first with native 64-bit support for the hypervisor itself.  While VMware ESX 3.0.1 does support 64-bit guest OS’s today, the hypervisor itself is only 32-bit.

The 64-bit support in Xen will be available as part of Xen 3.1.  Benefits to running the 64-bit version include native support for 64-bit applications (such as Oracle, SAP, MS Exchange 2007, and MS SQL Server 2007) as well as greater addressable memory (the 64-bit version raises the bar to a substantially higher 16 TB of memory, whereas 32-bit versions are limited to 16 GB).