Basically there’s three options, and the one (or more likely a mixture) we end up with remains to be seen.
If Microsoft or some hacker can get Windows to run on an Intel Mac (ie. write the extra bits needed to handle Apple hardware), then the Intel Macs will be able to boot Windows just like any other PC and many games should work ... as long as they don’t bang directly on the hardware to improve performance. This could be pretty bad for the Mac games industry.
The Intel Macs may well make it easier to port games since the code itself doesn’t have to be redone, again only any hardware problems would need to be fixed. This could mean a faster release of the Mac version instead of the long wait we often have now.
Similar to this is the fact that Microsoft should be able to make Virtual PC run faster than it does at the moment. It still won’t be full speed of a Windows PC, but should be much closer than it is now and make newer games more playable ... but only if they also fix Virtual PC to be able to use the Mac’s 3D graphics card / chips (which it currently doesn’t).