While Time Machine gained a couple new features in Lion, I've just stumbled across one (so far as I know, undocumented) new piece of functionality in Apple's backup software that will have a lot of users cheering: You can now use a previous Time Machine disk with a new computer and inherit the entire backup history of the older machine. And it doesn't even require a flux capacitor.
Given Apple's attempt with Time Machine to make backups painless, the migration process is pretty easy: Plug your old Time Machine drive into a new computer and open the Time Machine preference pane in System Preferences. Lion will remind you that you haven't selected a backup drive and will present a dialog box with a button to open Time Machine Preferences as well as a new option: Use Previous Disk. Clicking on that will provide you with a list of mounted drives that contain Time Machine backups.
Once you've selected the older Time Machine drive and told your Mac to backup, you'll be given another choice: Create an entirely new backup or inherit the backup history of the old drive. In the latter case, your old computer's Time Machine backup will be migrated to work with the new computer. Be warned, though—as Lion points out, time travel is a tricky business: If you migrate the Time Machine backup to work with your new computer, you won't be able to use it with the older computer.
This is a far, far better state of affairs than existed under Leopard and Snow Leopard. Previously, if you bought a new Mac, you could migrate data from your Time Machine backup while setting up your new computer, but after that, all those existing backups wouldn't be recognized by the Time Machine interface. Which meant either setting up a new set of backups running in parallel, or tossing that drive into a drawer and setting up a fresh Time Machine drive. (There were some methods for migrating a Time Machine backup, but it was an iffy proposition that required mucking about in the Terminal—and even then, it wasn't a sure thing.)
Now Mac users can theoretically maintain a continuous set of backups across multiple computers' lifetimes, which does a lot towards making sure that your old data never gets lost.