I'm running Windows 7 with the new taskbar, and Mac OS X Leopard on the same machine; my opinion is that they aren't really all that similar, except for superficial visual stuff. There are a lot of differences; for example, if you click on an icon of a visible application on the Windows 7 taskbar, the application is hidden. On OS X, the same action does nothing. OS X uses a blue oval underneath an icon to denote that an application is running; Windows 7 draws a box around it, makes the background behind the icon darker, and uses a shadow on the box to give the impression that the box is sitting on top of the taskbar. Windows 7 has a start button, OS X doesn't. OS X minimizes windows to a separate section on the right-hand side of the dock; Windows doesn't. The taskbar extends all the way across the side of a screen; the OS X dock doesn't unless you tell it to. Right-clicking the dock icon of a running application in OS X gives you a menu of commands that you can send to the application; right-clicking the taskbar icon in Windows 7 gives you the same old "Maximize/Minimize/etc/Close" menu that has been on taskbar items since Windows 95. You have to left-click the area to the right of the icon to see these commands in Windows . The Windows 7 taskbar can be configured to show the window's title beside the icon, so that it look and works more like the taskbar in prior versions of Windows; OS X only shows the name of the application if you hover the mouse over an icon. Windows 7 lets you represent each distinct window with a separate icon; OS X doesn't. You can put the trash can on the OS X dock, but you can't put the recycling bin on the taskbar in Windows 7. The Windows 7 taskbar has icons for sound, network and security; OS X puts these sorts of things on the menu bar at the top of the screen. Same with the clock. Oh yeah, and the Windows 7 taskbar can be placed at the top of the screen, but the dock can't. You can resize the OS X dock to any arbitrary size; you can't in Windows 7. The dock in OS X is reflective, the taskbar in Windows 7 has partial transparency but only if you are using the Desktop Window Manager. The dock is centered; the taskbar is left-justified; neither of these are configurable.
I could go on, because there's quite a bit more, but I think I've made my point well enough -- they really aren't that similar. About the only similarities I can see between the OS X dock and the Windows 7 taskbar, is that you click on an icon to start an application, and that icon becomes the running representation of the application. Otherwise, Windows 7 works more or less along the principle of taking the Quick Launch and the taskbar (both of which first appeared in Windows), and mashed them together into one thing.
