Seems like they already kicked Nokia in the face by giving HTC rights to call their 8X and 8S "Windows Phone" and basically saying it's their flagship at that event a few weeks ago. Wouldn't at all be surprised to see them make their own; they're already making their own tablet, so it's not like it's a big stretch.
It definitely benefits Nokia and MS to have more partners onboard with Windows Phone 8. I really liked the look of the 920, but I was worried it would never reach a critical mass of developer support with only Nokia building devices. If they could get one more major partner they could be a viable third platform, the way RIM was before they tanked. Unfortunately, Motorola is a no-go because of the Google acquisition, Samsung is enjoying great success with Android, and Sony is ... Sony. HTC already had a strong history of WP 6 devices before Android came along, and they're losing traction to Samsung in this space, so it makes sense that they would look to Microsoft for help.
Depending on how well it sells, MS might be solving the problem of finding a third partner by building a line of branded devices themselves.
Samsung is also producing Windows Phone 8 devices. Time will tell how committed they are to the platform, but I believe that a very large portion of WP7 devices were from Samsung.
