Apple just released iTunes 7.3 (34MB) which is required to activate iPhone service:
With iTunes 7.3, you can now activate iPhone service and sync it with your music, TV shows, movies and more. Also, you can now wirelessly share and enjoy your favorite digital photos from any computer in your home with Apple TV.
Early-adopters should note that iPhone also requires Mac OS 10.4.10. Now might be a good time to back up your hard drive, repair disk permissions and install that bad boy.