I bought the Sigma 24-135mm from Cameta back in Sep '06 - even before I got my K100D. When the K100D came, it had the same LCD shut-off problem with the lens. Sigma said there was no fix and suggested an exchange. Cameta was very good about it and promptly sent a replacement. The replacement lens worked well for a couple of months without any issues. But after I upgraded to the new 1.02 FW, the LCD shut-off problem came back. Not as bad as before, but it was happening once a while and was irritating me.
After reading OP's experience, I called Sigma and once I said 24-135mm and Pentax I didn't have to say anything else. They are totally aware of the problem. He told me to send it in for repair.
Shipped on 29th Jan - delivered to Sigma on 31st Jan
Sigma shipped back on 2nd Feb and received today 6th Feb.
Just one week from sending out to getting it back. Excellent service, I must say. Great job Sigma. The service receipt just says "Repaired Flex Board" - no other description. Tested the lens, works fine - hopefully the LCD issue is fixed for good. I am happy
Cheers
Teja