I've been running DirectRT on Mac Pro and MacBook Pro hardware for almost 2 years now, for behavioural and ERP studies, and can attest that DirectRT works perfectly on Macs running Windows through BootCamp. In this mode you are really just running Windows the way you would on any other Intel-based hardware - BootCamp just takes care of setting up your computer so that you can boot into OS X or Windows, installing drivers for the Mac hardware, etc.. - it is in no way an emulation or virtualization. Parallels (and VMware Fusion) are however virtualizations and cannot be expected to be as reliable as you need for cognitive research. Rebooting between Mac and Windows OSes is easy anyway, and it's not like you're going to be checking your mail or doing anything else in Mac OS while running a subject anyway

The only oddity I've experienced is that on the MacBook Pro (but not on the MacPro), there seems to be some weirdness with the video card such that DirectRT runs at whatever screen resolution that Windows is set to, not the resolution you specify in the Styles file. But I haven't updated this machine's BootCamp to the current version that ships with Leopard (which has updated video drivers), so this may be fixed as well. At any rate it's a trivial problem really.