The conditions to create an Osborne Effect are not met, IMHO:
- the FF doesn't succeed any of the Pentax DSLRs, but it marks entering into a completely new market segment, i.e. above APS-C and below 645.
- thus, it's priced out of the reach for most buyers (for whom APS-C will continue to be a more compelling proposition)
- and it's actually not just a camera, but an entire product line - Pentax already started selling (the 150-450mm reached the market).
The only way an Osborne Effect could be created is if dealers would empty their shelves of the APS-C Pentax DSLRs, to exclusively go with the future FF model. Basically, they would have to start believing that everyone could suddenly pay for a camera which is (let's say) double the price of the current APS-C top of the range.
If that would really happen, I guess we would be able to perfectly correlate it with a global spike in LSD consumption
Realistically speaking, the only (short term) negative effect could be some reduction in the number of high-end APS-C products being sold, followed by a spike in FF sales. Positive effects should not be ignored.