Both Arne Hess over at
the::unwired and
engadget mobile are speculating that since Orange and HTC are involved in this Google Phone venture, it is possible that Google's phone could be running on Windows Mobile with special Google software to enable search, location based services and other stuff that Google does really well.
If Google were to go with Windows Mobile OS, then would benefit more if they make the software available to all the Windows Mobile users, instead of just sticking with one carrier or manufacturer. That is why I think the phone will have a more proprietary software and hardware.
The only reason why I would think they would want to partner with one carrier (at launch) "and stick with a HTC Windows Mobile phone" is to deliver on the promise of faster surfing/search and Google services; this I believe can be achieved by having a massive array of Google servers that cache the search data and services right on the Orange phone data network. I remember reading about this someplace (will have to dig up the article).