Delayed Steve Ballmer announced another possible shipping delay for Vista yesterday - now probably Feb or March. Can you imagine how many beta releases there will be between now and then and how neat it would be to try them out but ultimately discard while our Windows using colleagues have to come and see our Macs to preview the next Windows...
Vista requires ACPI support; Parallels does not support ACPI. So, in a word, "No". Hopefully PW will get ACPI functionality for exactly that reason...
The question would be better rephrased to 'would Parallels support ACPI?' IIRC, the dev team said that it will eventually, but not in the GA (i.e. official 2.1 version release).