Wow, after saying ever since the Intel announcement that Windows might run on an Intel Mac but that Apple wouldn't support it, Apple releases Boot Camp, a tool to aid dual booting OS X and Windows XP. Now, they still don't support Windows but they are providing a nice tool to handle the partitioning, A Startup Disk control panel for Windows and, most importantly, Windows drivers for all the hardware including the little stuff like the eject key on the keyboard. The partition tool actually resizes the existing single partition which is pretty great all by itself.
Even with the Apple provided drivers, not all Mac hardware is supported when running Windows. The biggest disappointment is probably no support for integrated iSight cameras but this may be a temporary situation.
Ultimately this will be included with Leopard, OS X 10.5, but I think this beta release will still stir up the rumors (as well has hopes & dreams) of support for virtualization, allowing you to run OS X and Windows side-by-side. Even if it does, it's possible it won't support true virtualization on the current Intel processors, Intel (and AMD) are coming out later this year with processors that specifically aid the operation of multiple operating systems simultaneously. Mac virtualization would be a nightmare for PC manufacturers because even if they (or Microsoft, perhaps using the now-free Virtual Server) could run OS X on their hardware, Apple's licensing doesn't allow it. Sure, some people would do it anyway but it wouldn't be supported by any major manufacturer or Microsoft.