I've asked the same question, and searched for apps that can open a tiff and so far NOTHING. Really sucks, I like this phone so much more then my blackberry but I need to open tiff to view faxes.
TIFF is really just a wrapper format and has hundreds of different formats that can be encapsulated in it, has plenty of exploits, is mostly depreciated in favor of PDF (although I still know quite a few office systems that use it), and is further associated with another depreciated method of communication, faxing.
While most offices have moved on to PDF and email, I know 90% of American ones still haven't (mine still hasn't). Google probably just didn't see the need for it.