Windows Live Photo Gallery (WLPG) stores its metadata in XMP, or Extensible Metadata Platform. XMP is a standard way of storing image metadata, often called a "sidecar", and works with all forms of images including RAW. There is a good explanation of how WLPG stores their facial tagging records in XMP here. Given that its stored in XMP, it should be possible to import that metadata into iPhoto, as I'm pretty sure it supports import from XMP at the very least. I can't say for sure it will support the facial tagging structures...but all the rest should import without problem.

The page that descrives how WLPG stores facial recognition data should provide enough information for someone to write a converter for it. It seems pretty simple and strait forward...at least for tracking the regions. I can't say how iPhoto's facial recognition engine works, so it might function completely differently, and be entirely incompatible with Microsoft's data. If not, then all hope may not be lost, so long as you keep the XMP sidecar files around with the original WLGP region data.