we beg to differ that sync/sharing isn't a killer app ;)
My guess is you want to give everybody what programmers have been able to do for years with distributed VCS, publicly addressable computers and rsync. And presumably want to do it well enough that even programmers who could get it for free wouldn't be able to fool themselves that paying wasn't worth the convenience.
Now what Microsoft is doing is probably connected with there life-logging stuff. MSR has been going on for a while about how we'll never delete anything and every document and every phone call we make etc will be recorded and searchable. The mesh is presumably the infrastructure that all these Windows Mobile devices would use to make this seamless.
However this is insane, there are already more mobiles in Europe than people, and Microsoft can't even keep XBox Live running. Just have the devices query/talk to each other without getting some central server involved.