A recent Apple technical note lists which Intel-based Mac models can run the 64-bit editions of Windows Vista. To little surprise it's the 2008 flavors of Macs for professionals: the Mac Pro (Early 2008) workstation, the 17-inch MacBook Pro (Early 2008) and both Early and Late 2008 15-inch MacBook Pro notebooks.
However, it's a different story when we look outside Boot Camp.
VMware Fusion supports 64-bit Windows Vista as well as the 64-bit edition of Microsoft Windows XP Professional (and a bunch of Linux and Unix OSes). The hardware requirements are "Intel-based Mac with a Core 2 Duo or Xeon processor," which greatly expands the list of usable machines, including many non-professional iMacs and MacBooks as well as the MacBook Air (and even several models of the Mac mini).
Here is a place on the comparison table where VMware Fusion differentiates itself from Parallels Desktop; the latter currently only supports 32-bit OSes. Now, Parallels Server for Mac can run 32- and 64-bit OSes, however, this runs on Leopard Server and costs $1,248.75.
So, for most of us, VMware Fusion scores in the 64-bit department.