Rafael will document how to "publish" built-in XP applications so that they appear under Windows 7 with XP Mode (apps installed after the fact are auto-published), but here's a fun shot of IE 6 (XP) and IE 8 running side by side under Windows 7:
Good stuff.
(Note that, as you can see above, XP apps aren't [currently?] able to use Aero Snap, so I had to manually position that window.)