Tokina and Pentax have collaborated on several lenses, apparently with Tokina taking the lead on several of the projects. And there is a Tokina 35mm ƒ/2.8 Macro lens—the AT-X M35 Pro DX Macro—for Canon and Nikon only (what that means is, no Pentax) with the same specification as the Pentax 35mm DA Macro.
As you can see, the published block diagrams for the two are very similar:
(Pentax above, Tokina below)
...And although the diagrams aren't identical (there are subtle differences in several places, especially in the cemented 3-4 doublet) the similarities certainly indicate some sort of sharing of ideas at some stage. People tend to assume similar designs are "identical" if they're close, and this can be a mistake—a cross-section only shows partial information, and very subtle differences can be quite significant. The above looks like more than coincidence, however.
I received a strongly-worded message from my contacts at Pentax yesterday stating in no uncertain terms that the 35mm DA Macro is purely Pentax's design and manufacture, from concept to market—that is, the project was 100% Pentax.
The Tokina lens certainly looks different!
Despite the disavowals of any direct connection in this case, the cross-sections are close enough that if I shot Canon or Nikon and wanted a very high-quality normal fixed macro, I'd put the Tokina very high on my short list to try. It may not be the same lens, probably more like a cousin than a twin, but hey, if it walks like a duck and quacks like a duck....