I have (almost) the same question regarding Google Earth and HP ScanPro software...to get them to use SeaMonkey for E-mail instead of their choices of Mail, Eudora, and Entourage.
Isn't it odd that OS-X is not egalitarian enough to allow the user to choose a default E-mail app and make it work system wide ? I have no use for Apple's Mail , or Safari for that matter, but you still need to go into their Preferences to default out to another App,
I didn't realize AppleScript-ability was the determinant for iPhoto, which I also do not use...