I was having trouble placing a some tiffs and not others into an ai-CS file. I save all my tiffs with a LZW compression, IBM PC Byte order, etc. but was getting the error "That Tiff file format is not supported." Similar problems for a PSD file. Aha! Mystery solved. The images were in LAB color mode (converted for optimizing an Unsharp mask routine). Returning to RGB fixed it. Hope this helps someone.