With the latest version of iPhoto (maybe an earlier version but I didn't check it out) iPhoto will create the EXIF Date Created field in the file and add the date you enter if you use the Photos->Adjust Date and Time menu option.
If you select the option to Modify the original files the modified file will also have the date embedded immediately and you don't have to export it with the metadata option to get in the modified version.
HOWEVER, none of this works with tiff files - at least I've not been able to get it to work with them.
I can get EXIF File Captured dates into tiff files if I convert a jpg or Photoshop file with such a date to tiff. But not with iPhoto.
That's a good feature request via
http://www.apple.com/feedback/iphoto.html.
OT