We've been hosting Exchange and the rest of the Microsoft stack for clients as long as it has existed. The only thing cloudy around BPOS is the billing mechanism and baffle-them-with-bullshit technology oriented sales pitch for what is really a substandard service offering. When you add the wrapper (specifically L2) back in to what they provide, it usually winds up being more expensive to go with BPOS than a traditional outsourcer. That's what we've seen with the bigger clients at least... and assumes that Microsoft isn't giving their licensing away (which is something we've suspected in a few cases).
The sad thing is that most clients don't really understand what they're signing up for until it's too late and financial penalties are involved if they want to back out. Our clients wouldn't let us in the door with the shit T&Cs in BPOS, but everyone runs Microsoft so they can get their foot in the door anywhere... and by the time they're done throwing out this low Low LOW price offering and talking Mississippi and clouds and automated provisioning and pay-by-the-drink most clients think this really is something new and revolutionary where it's really anything but.