Ask Joe Belfiore himself, ask if MS would use a dalvik compatibility layer any time soon...

It seems very unlikely MS would be tempted to do that given:
(1) The axing of Nokia AOSP (w/MS services integrated)
(2) The plans for Win/RT/WP that pre-date these ACL/AOSP rumours that aren't yet fully executed (sounds very Nokia-esque! ;-P)

Maybe when (if) they're in a much direr position & their fully executed plans clearly aren't helping, but not now.
AOSP for the top-end (w. integrated MS/Other services) is a possibility, maybe a limited set of devices there, but still unlikely.