It's important to note that this is from a leaked copy of the Longhorn build that developers will be receiving Monday at MS's Professional Developer's Conference. The prevailing wisdom seems to be that although this contains elements of the eventual UI, the "Aero" interface-of-the-future is specifically NOT included in the build. There's supposed to be a demo of it at the conference, though, so more info might be forthcoming this week.
All that being said, it's obvious that MS has been experimenting with (some would say "ripping off", but I'm going to avoid that terminology for the moment...oops, too late) many of the same design elements found in other products. The new application skins definitely bear a striking resemblance to a darker version of the OS X brushed metal, as many have already pointed out. The back-arrow on the IE screenshot looks almost identical to the back arrow in Mozilla's "Modern" theme (which is sad, really -- although the concept of "circular button with left-facing arrow" is hardly original or unique to Mozilla, there are endless ways to differentiate the details; did the arrows really have to be *exactly* the same shape and relative size?)
Despite everything I've just written, I'm actually willing to give MS the benefit of the doubt on this for a while. In the screenshots that have been posted, nothing is finished -- most icons, buttons, etc. haven't received any treatment yet, and are still the garish XP Luna look, or even carry over from Win2k. And "borrowing" UI elements is nothing new for Microsoft -- but it isn't new for Apple or just about anyone else either. If in the end MS manages to 1) keep the UI familiar enough that their user base doesn't need substantial retraining, 2) "borrows" some successful or even "cool" UI elements from other sources, and 3) hopefully comes up with at least one or two original ideas along the way, then they will have succeeded.
I'm mostly interested in the demo of the Aero UI. And what changes they'll make in the next 2-3 years before Longhorn actually shows up on store shelves.