Count me in the crowd that thinks that K-5 bounce P-TTL flash is very broken.
Why?
Off-camera wireless P-TTL works fine, even when bounced. I have experimented, setting up the camera exactly the same way for two shots, with the only difference between the two shots being as follows:
In shot 1, flash is mounted to the camera hotshoe and bounced
In shot 2, flash is taken off-camera but bounced at the same approximate angle, using P-TTL optical wireless mode (controlled by the on-camera flash)
Repeatably, shot 1 is badly overexposed and blown out. Shot 2 is fine. The two shots (on Av) have the same ISO/aperture/shutter, but I can hear (by the "pop" sound) that the flash in shot 1 is firing at a much higher power.
This is using a K-5 on latest firmware (1.13, right?) and an AF360-FGZ flash. I have also tried different lenses (Sigma and Pentax), different flashes (a Metz in addition to my Pentax), and different bodies (K-x works fine bounced with the same flash).
It's the camera, it's defective, and it's annoying.