From what I have read and talked to ESRI support, ArcGIS installs fine through Parallels or VMware. Basic processing and functions work without any problems, but some users report that more demanding geoprocessing tasks cause performance to become sluggish. Another option is to run WindowsXP through Bootcamp. Users report that there are little to no problems in performance using this method. However, if you want to use Mac based Adobe CS3 to handle any exports from ArcGIS, then you will have to reboot and enter OSX, which can be a hassle if you like to switch back and forth between OSX and XP.
A bit off topic... well, maybe not? Is it feasible to run both Bootcamp (ie, XP native) and Fusion (XP in a shell) on the same machine? would you be able to utilize the same install of ArcGIS, or would you need two separate installations in order to access it from both environments? I could see where it would be beneficial to be able to boot to xp for the hardcore stuff, but use the Fusion-ed version for lighter duty tasks and convenience.