Jeff
Anyone who has CS5 is currently under an NDA, so any answer can only be in general terms or based on guesswork.
That said, MB is mainly for those Photoshop users who have never liked the idea of Photoshop losing its file browser and it becoming Bridge, and that's mainly where the hype's coming from. It also reflects how a lot of users of multiple Creative Suite applications and how a lot of graphic artists like to have little palettes containing their various media assets (similar tools are in Extensis Portfolio and iView). Some Lightroom users might like it, but more because they are also users of the Creative Suite.
John
my edit - clarity