I've just got an SMC A 50mm f1.7 and an Auto Chinon 28mm f2.8 lens (which lacks the A setting) for my K10D, and have been playing with them taking photos of the kids this evening, which requires a flash.
I've found that the SMC A lens works well in fully manual mode (M) or flash sync mode (X) with the internal flash on the K10D. The fully manual lens, which would be the equivalent of a Pentax M or K series, does not work so well with the built in flash, and seems to fire on full power at all times. This means that you have to manually adjust the aperture of the lens to balance the exposure and certain combinations of short range and shallow DOF are not achievable.
I do have a Cobra 300D flash that is a TTL unit, and this works fantastically with the manual lens and K10D even down to short range and f2.8
. I just mention this as a word of caution that if any wants to go down the manual lens route, and will need to work with a flash, don't rely on the built in unit, get a TTL unit!
All the best,
Nick.