I bought this lens brand-new from B&H ($250, but the only way I could find a copy that didn't have dust that interfered with macro shots) more than a few months back and used it on and off with my K100d with no real problems that I can recall. I just got a K20d last week but really didn't have time to finagle with it until tonight. Shots with my Sigma 17-70mm & my Pentax K 50mm 1.4 came out fine, but when I popped on my Tamron and took a wide-open shot, I was getting pics like this:
(full-size crop-iso200 at 210mm at f3.5)
And this shot was
after I cleaned both my lens and my UV filter with a new lens cloth, and re-mounted my Adaptall-2 adapter to make sure the aperture was being properly operated (and the blades were clicking fine, too) and refocusing. The diffusion-effect wasn't visible through the viewfinder, so I don't think it was the light (400 watts of Reveal tungstens in a fixture almost directly overhead). Using the lens hood didn't change anything, nor did using a paper to block some of the light. I started taking shots while stepping-down, and the effect lessened, and was pretty much gone by the time I hit F8 (1/2sec at iso200):
So I'm debating whether or not I'm either missing focus by just a bit (the "I'm a yutz" part) or somehow something got screwed up in the lens (which would be really annoying, considering I don't abuse my stuff).