I came across this article randomly.
LumoLabs
In its conclusion section it mentions that
"The K-5 as it presently ships has a flaw in its phase detect autofocus module or software which causes it to front focus in low enough light. This applies to both v1.02 and 1.03 firmware releases."
Not sure about validity but it makes an interesting point that even with firmware update the AF issues still exist.
I own a K-5 too and I randomly see AF issues particularly in low light (with and without tripod) and multi-segment AF setting. I use DA* lenses (50-135) and not sure if it was lens or K-5 itself.
Any comments/opinions/experiences?