I wouldn't call the Pentax K5 lack luster, it's probably the best general purpose aps in the market.
I remember in the '70's cameras like the K2, ME and MX were so beautiful you could put
them in a display cabinet and just look at them !
But....they weren't as robust as Nikon, and Canons in those days were.......well just a bit plasticy really
Just after the K7 was announced I thought I'd try a Pentax as a small lightweight travel camera, so bought the much cheaper K20 which had the same chip to try it out. Boy was it lousy, talk about poor DR and general latitude on anything above ISO 100. However to give Pentax credit they quickly secured the new Sony chip and produced the Sony powered K7 - the K5.
Looking at Pentax pricing, if they did introduce a FF I don't think it would bs much under the D6 - 600D initially, but then would drop substantially, and there's the problem. Pentax have had to really drop the price of the very good K5 to tempt buyers away from Canikon, and so I don't think the company will see it as worthwhile.
And the problem is really - everything about Canon works well, from the bodies to lenses to software etc. If I bought another Pentax it would just be a nostalgic purchase.