UPDATED 2008-11-22:
So, you have a photo in Aperture 2, and you need the date (and/or time) adjusted. It’s actually pretty easy once you know where Aperture has hidden the option! See further below for info on changing multiple images.
Select the photo, choose Metadata>Adjust Date & Time from the menu.

Type in a new date and/or time. Select the checkbox if you also want to update the master file as well.

Take a look at the Metadata panel, and the Image Date has been changed.

You can also adjust the date & time for multiple photos at once. But it doesn’t work exactly how you would think. It actually works better… once you understand the concept!
Select all the photos.
The date/time that appears in the change window, is for the primary selected photo. Make your adjustments, and Aperture 2 will adjust every selected file. The cool thing is it will adjust according to the original information.
So that means, if you have a series of three photos, each taken 1 minute apart from each other, Aperture 2 will adjust accordingly. Each of the photos will still retain the 1 minute difference. Cool, huh?!
Video tutorial on Apple’s web site