While I don't consider this is a loss of a differentiator of much significance, Microsoft does/has promoted XBL integration as one of the key features of WP. What's more disturbing is that Microsoft isn't putting all their resources into creating NEW differentiators for WP. When I see all the effort Nokia is making in promoting WP compared to Microsoft is, it really does seem like Microsoft is trying to make WP fail. Microsoft is the proverbial chicken contributing eggs for breakfast, while Nokia is the pig expected to "contribute" the bacon.
Microsoft is probably reorganising internally to get everyone ready for WP's eventual move to NT. I wonder how long it'll take.