Ok--I have a similar problem (G5 iMac, OS X 10.5). I can open the
tiffs in Preview, but iPhoto will not import. When I try to save out
as something different in Preview, the only options I have are tiff
(which, when saved, iPhoto will still not open) and pdf.
I have previously had no problem opening tiff files in iPhoto or
Aperture (2--no Intel Mac yet).
Any clues?