Dunno if that's what you've seen, but PCs have had programmable (through I²C) PLLs for a couple of years now. There are some utils around that allow for changing FSB speed (within certain limits) while the machine is running.
I don't use them any more since I rather o'c through multiplier settings - my Athlon XP 1500 identifies as XP 2300 (1866 MHz)... :lol:
