Seberry, this is in camera. No problems with card reader, either external or internal.
Ash - Honestly, I hope it's my card (s) - sending my camera to Pentax for warranty repair is a heck of a lot more stressful than spending a couple of hundred on a handful of new cards.
But there's my problem - while the assumption being taken here is that it's a card problem, my approach instead is to be 100% sure it's not the camera.
Earlier today I shot a batch of test shots and no errors. Tonight I had a studio session, over 140 shots, not a single error message. I was ready with both my backup camera and extra cards just in case but there was no need.
Tomorrow I will use the card which gave me the error message in the first place and shoot a batch of nature shots in my backyard, see what happens.
As I said, I am going to replace my cards as I know I've rolled the counters on the K10d at least 6 times, and at least 3 times on the K20d. Add to that the read writes from putting them on the computer, backing up files, moving files, deleting files... I'm sure they've been beaten up.
But if I have to send my camera in, I have to do it next week to make sure it gets back before the repair center goes on break and general drunkeness ensues in the Rockies.