This is the article you guys should be looking at:

http://en.wikipedia.org/wiki/Generation_loss

In short, like pjz99 said, in some cases JPGs can work fine with multiple saves, in other cases they don't. The "don't" bit is usually when you edit the image, scale it or crop it etc, basically any change that causes the compression process to change increases the artifacts. Frankly it's just not worth the hassle and it's far easier to just use a lossless format instead.

As for the 100% thing, i'll just quote this bit from that JPEG article posted by Haarspalter:

"Since the quantization stage always results in a loss of information, JPEG standard is always a lossy compression codec. (Information is lost both in quantizing and rounding of the floating-point numbers.) Even if the quantization matrix is a matrix of ones, information will still be lost in the rounding step."

For the visually inclined, there are pics at both articles to see for yourself.