Looks like the market is being flooded not only with a host of compatibility layers, but increasing number of true virtualisation applications, or rather, hybrids between the two.
To drop some keywords, we have Rosetta, Wine and its various Crossover derivatives, VirtualPC, PearPC, qemu and Q, VirtualBox, Xen, Parallels and VMWare. Clearly a lucrative market.
I have been interested recently in the increase of usability, including true drag and drop in products by Parallels and VMWare (1), free-floating windows (called Coherence in the Parallels product for Mac), and I would like to see the window manager being fully replaced with the native one, which even Parallels does not seem to have managed yet, judging by the screenshots (but it can be done using WindowBlinds in Windows - $19 on top of the $79 for Parallels Desktop; VMWare pricing not known yet). Arguably, there is some benefit in having a slightly different window border style for Windows windows (ha!) in order to identify them quickly (and here).
But I have also been interested in seeing how little people seem to expect in terms of hardware support or clarification thereof. For instance, I've always been dismayed that there was no equivalent of VNC that would also transmit audio. Similarly, with Windows Vista being the first consumer OS going to market with true voice recognition (OS X 10.4 "Tiger" has a matching algorithm that matches up your voice with any of a small number of known commands, useful but not the real McCoy) - the killer feature in my opinion - it is not clear whether this will work in any of the existing virtualisation products!
(1) VMWare, iirc, has yet to release a product with their announced drag and drop functionality, but for Parallels it is available in a beta version, with hints of a release in the very near future; both on the Mac, not sure about other OSes.