In OS X all the windows are rendered off-screen. What is displayed on the screen is a compisition rendered as one big window.
Each window, that you see, is pretty much one simple rectangle primitive in 3d space with the texture from the pre-rendered window mapped to it. So all that is needed is to run a single OpenGL-based transformation to resize the windows to miniture sizes.
Microsoft has duplicated this off-screen rendering technic and it will be aviable to users with fast hardware in the form of "Areo Glass", which is what the highest level of eye-candy will be called in Longhorn. Or something like this.
Now I doubt that MS will replicate the Expose' feature to Longhorn users.. that will just make them look stupid for such a blatent copying of a Apple feature, but I wouldn't be suprised at all that you could get something like that in a form of a theme pack or some shareware program somewere.
In linux you can do this too if your using experimental software. Recent versions of X.org's X server supported 'compisition extension', which is what the off-screen rendering stuff is called in Linux, but the software is still rendering in a conventional 2d manner, not in OpenGL, so it's a bit slow.
There are OpenGL-based versions of X.org's X server, that are experimental, that are OpenGL based and a couple people have whipped up hacks to recreate Expose' effect on that just for the hell of it, but for the most part nobody cares just yet.
And while comparing OS X Dock vs Windows Taskbar keep in mind that OS X and Windows handle windows completely differently.
With OS X each application has it's own sort of 'virtual desktop'. It it's own space with it's own windows, and the application menu (file, edit, help, etc, menu entries) are at the top of the screen, always.
If you open 10 browser windows, they all layer in their own space. Then if you open up a word proccessor program it creates a new 'layer' and puts it in their.
So you click on the word proccessor program then all the other non-related windows go the back, you click on a browser, then all the browser windows come to the front. So on and so forth. Each application has it's own space.
So in OS X's dock you have a entry for each application, not for each Window.
In the Window's desktop every window you open up gets added to the general mish-mash that is your desktop. All of them exist in the same space.
You open up 10 browser windows, and a few word documents you end up with a situation were you have a big mix-up and it can be difficult to find what you want. It only has one desktop...
It works fine for single-window'd apps to a certain extent, but if you get multiwindow'd apps.. like Gimp for isntance, it's very easy to get confused between all the different windows and menus and such.
That's why apps like Photoshop, that use lots of small windows, have to create a big 'parent' window to contain all the little windows. In this way it tries to emulate Apple's way of handling windows on MS Windows.
I personally prefer to have windows grouped according to application like OS X vs the general mish-mash like MS Windows.