To be blunt: no, there's no legacy switch in Windows 7. You'll be using the new taskbar, and there's no going back (apart from installing a previous version of Windows, of course). "Sinofsky told us that there will be no ability to enable the old taskbar since, in Microsoft's opinion, the new taskbar's leap in usability negates the need for a 'less-able' option." Apparently, the massive changes in Office 2007 gave Microsoft insight into how users respond to aggressive changes to familiar user interfaces, and it concluded that users generally took about 48 hours to 25 days to see productivity gains when using the ribbon. Since users spend more time using the taskbar than using an individual Office application, Microsoft reasons that the change should be less hard to adapt to.
Jump lists do not only provide quick access to commonly used files and features, Sinofsky explains, but they also allow programmers to "stop annoying users". Sinofsky points to Windows Live Messenger, which has a normal window, an icon in the system tray with a context menu, and "bizarre balloon messages if you close it, reminding you that it's still running." The jump list allows programmers to put everything into that list, and keep the system tray icon to do what it was supposed to do in the first place: notify users.
Ars, rightly so, points out that it is Microsoft's job to actually lead the way with these new features, and properly use them in its own applications so that other developers can see how it's done - something Apple is pretty good at.