- Download iTunes 6.0.5. Leave the downloaded package alone for now.
- Control-click your copy of iTunes 7 (probably in your Applications folder), and choose Show Package Contents from the pop-up menu. Open the folder Contents -> Resources. Leave this window open.
- Use Pacifist ($20 shareware) to open up the installer you downloaded, and navigate to Contents of iTunes.mpkg -> Contents of iTunesX.pkg -> Applications -> iTunes.app -> Contents/Resources.
- Select all the icon files (.icns) in this folder, and drag them from Pacifist to the Finder window you left open. Confirm the replacement.
- Log out and back in again.
[robg adds: I'd also recommend quitting iTunes before changing its package contents...]

