The possibility that the final sensor will be drastically different from the beta camera sensor is unlikely. Perhaps I'm just trying to be realistic, but I think the K-7 will have essentially the same image quality as the already very good K20D.
However, just like proofreading a story before sending it to print, the possibility is good that there will be minor tweaks between the beta camera and the final production version. A detailed review that notes minor flaws could be affected by these minor tweaks, and therefore it is only reasonable to wait until a final production version arrives before doing a review. You wouldn't want somebody to do a review of a book filled with typos, right?
The disclaimer is really just projection (Edit: I meant protection, but I think this sort of proves my point
) for AP to those who would complain about the wait.