Thank you again for all the tips and links relating to wide lenses :-)
Bob Atkins has written a nice introduction called: "Wide angle lenses
for small sensor APS-C Digital SLRs" that compares the new lenses
from Tamron, Tokina, Canon, and Sigma. A second page adds thoughts on
fisheyes. He also offers a review of the Zenitar with example photos!
http://www.bobatkins.com/photography/reviews/wide-angle-lenses.html
Not many of the new APS-sized wide lenses are actually available yet,
especially not in a Pentax mount. This includes the Pentax 12-24mm
that was announced in March (I was wrong there).
Does 1-2mm matter? I don't know, I guess other things will influense
your choice more than whether the lens is 10, 11 or 12mm at its
widest. What do you think?