If this hasn't already been said, do make sure you clean the o-rings, and make sure there is no sand or salt encrusted on them. Letting the camera sit in the sink in fresh water for a while after using the camera helps to avoid this. A dab of silicone lube to the o-ring will keep it in good condition and help keep out water that might be seeping past the ring, and humidifying the inside of the case. Also examine the pass-through button o-rings for signs of grit and get in there with some lube.
If the strips are not working, there is some seepage going on, without a doubt. I use my GoPro 2 in Florida, the humidity capital of the friggin world, and I never had an issue or needed strips.
Another issue that can cause fogging is taking the camera out of your air conditioned car into a warm environment.
Also you can treat the inside of the lens with Rain-X Anti Fog, which you can find at Wal-Mart or any auto chain store. Normal Rain-X works great for the outside of the lens, so you don't get that water drop that hangs in the middle of the shot through 20 minutes of video. It's great for your sunglasses, too.