2012.02.04 19:15 "Re: [Tiff] ready for 3.9.6 and 4.0.1", by Lee Howard
What I've ultimately decided to do is to go ahead and make the "tiff" package based on tiff-4.0.1, but the "libtiff-dev" virtual package will remain only on libtiff4-dev, which will now be in the tiff3 source package. So the tiff3 source package will include binary packages libtiff4, libtiff4-dev and libtiffxx0c2 only, and the libtiff4-dev will "Provide" libtiff-dev.
It's all so clear now! ;-)
(But seriously, I have the most impossible time figuring out Debian versioning. Every time I have the privilege to install or work on a Debian system I spend way too long trying to figure out if I should be apt-get installing "libtiff", "libtiff4", "tiff", or some other unknown.)