Sheets are the new, non-modal file save dialogs.
I think it's a combination of developer laziness (i.e. not using CoreGraphics text rendering and sheets and X interface elements in MS products) and also the lack of standardization across API's.
Carbon apps, even when programmed as Mach-0 and using all the X-specific tech available to them, at the current time, will not look exactly like Cocoa ones. Non-transparent sheets, services... etc. Also, as a lot of 9 apps use Geneva as their small font, and true X uses Lucida Grande, so many carbon apps won't look right without a major interface overhaul.
The one thing that really peeves me about X is that there is no true interface consistency across anything other than Cocoa Apps. I think, probably, when 9 is nothing but a memory (hopefully by this time next year) we'll be seeing a lot more mach-0, totally redesigned X interfaces, and hopefully Apple will update the carbon spec (now that 9 compatibility can be sacrificed) to make things like transparent, animated sheets, drawers, and CoreGraphics easier for developers to access and implement in their carbon apps.
It seems that originally, Apple wanted carbon to be a "bridge" and more of a temporary API. Now, it seems that Carbon needs to mature very quickly into a true X api that is 100% the equal of Cocoa: right now, it's probably about 90% there, but it still needs spit and polish. On the other hand, Cocoa needs to have some rough edges worked out on its end, too.
Basically, I think that, in about a year, X will be a very different experience altogether. Apps will look like true aqua, everything will be truely pre-emptive, (no more menu calls locking the whole app) and easily distinguishable carbon/cocoa apps will be a distant memory.
[ 12-16-2001: Message edited by: Jonathan ]</p>