Despite the fact that there is no better protection around, I am finding that I am less and less inclined to pull on armored leather pants for general street and canyon riding. I own a couple of pairs of Kevlar riding jeans and I like them. However, I sometimes question the safety of the armor in loose-fitting riding jeans. If the armor doesn't stay in place, it can't protect.
The Icon Field Armor Knee/Shin Guards are a nice option for protection under your everyday jeans. They do have vent holes in the plastic and a mesh chassis to flow some air, but they obviously will still retain heat. However, hot and protected is better than the alternative.
I have found some independent shin protection to be a bit bulky. The Field Armor guards have a substantially lower profile than most other knee and shin guards I've seen, so they are less restrictive and less noticeable under jeans than other types.
If you are looking for lower body protection that is versatile and unobtrusive, these Icon Field Armor Knee/Shin Guards are worth a look. Make sure to look at the sizing suggestions carefully – calf dimensions vary greatly. The Field Armor Guards retail for about $65.