The PDF Association and TWAIN Working Group have announced a partnership to develop a specification called PDF/Raster or PDF/R. It’s described as “a component of TWG’s TWAIN Direct™ initiative, a language/protocol that eliminates the need for users to install vendor specific drivers as communication between scanning devices and image capture software applications.”
I’m not sure what need this is supposed to fill. PDF can be used as an image format, but it’s a rather complicated one. Pamela Doyle, Chair and CEO of the TWAIN Working Group, says:
The TWAIN Direct language is designed to revolutionize how scanners and software communicate. PDF/R, an integral part of this language, provides the portability of PDF while offering the core functionality of TIFF. PDF/R is a strict subset of the ISO-standardized PDF file format for storing and transporting raster image documents, especially scanned documents.
This suggests that TIFF isn’t sufficiently portable but doesn’t explain why. Maybe what concerns them is the entropy which has overtaken TIFF. The current specification (6.0) is dated 1992, and to use it properly today you need to know about three technical notes, a large body of third-party tags, and a certain amount of folk wisdom. On the other hand, the PDF Association tends to make extravagant claims about PDF, calling it an application platform and portraying it in a superhero suit, so a certain amount of bias might be involved.
Other image formats are available, and this attempt to bring PDF into the image format market is an odd one.
Mad File Format Science 
Hi Gary – I recently started following your blog (found you by way of the TI/A effort I think). I’m one of the authors of the PDF/raster draft spec and I’d be happy to discuss technical details, history, rationale, etc. I’d like to share the draft spec itself with you too. We were planning to publish it as an Internet Draft but still have a couple of holes to fill.
Please do discuss those things. If you like, I could arrange for you to do a guest post, or I can link to whatever information you provide.
As for reviewing the spec … Everyone would like my input but nobody has money to pay me. I’m afraid that at present I have to focus my time on potential income-producing activity and not spread myself too thin, but thanks for the offer.