You can mix PC133 and PC100 with no problems, but it will run at 100MHz, not 133MHz.
If you put PC133 RAM in a PC100 board, it'll run OK. I've never tried putting PC100 in a PC133 board though. With today's failsaves and moron-proof technology, I'd assume that the FSB would revert to 100MHz if you put any PC100 in a PC133 mobo along with PC133 RAM... the PC133 RAM would go to 100MHz to follow suit.