Hi Mike,
I have two pairs. My Draggin' Jeans have kevlar down the outside seams of the leg, over the knees, round the bum, and in the hip area. I'm not entirely convinced they would work very well if you slid for any distance (unless you were sliding on your arse); I suspect they would rotate on the leg, and thus bring a non-kevlar bit into contact with tarmac. My other pair are from a company called Grinfactor, and they sell them as 'paranoid' jeans. They have kevlar in the same places as the Draggin' jeans, but then they also have a second kevlar lining, of the entire jeans. So even if they rotated, there would still be kevlar between you and the road.
I didn't buy them because I was paranoid - I just wanted something which would be warmer than the Draggin' ones, and thus could be used more of the time when the weather's cold! The only downside to the Grinfactor pair is that the velcro used on the pockets that hold the knee protectors can be scratchy.
Regards,
Martin