as long as apple service installs it, the warranty will not be void. they are capable of telling you if the part you want to install will break the unit, and it is their responsibility to warrant their installations.
i have had 7200rpm hard drive in titanium powerbook before and now in this 1.25GHz albook. wonderful performance, just go for it. nothing to worry about, but the cost of the repair (drive exchange) operation. i changed it myself for the titanium, but had service change it for my albook. it cost me 50 euros in here, don't know how cheap it is over there 😉
(in the titanium, i first noticed that the 7200rpm drive runs MUCH cooler than the original 4200rpm drive. the change didn't affect battery life at all. it just was much faster and the cooling fan seemed to only function when the cpu levels were constantly over 60% - compared to operating almost always with the 4200rpm drive. the reason for this: 7200rpm drive gets the job done faster so it gets more idle time compared to 4200rpm drive. work cycles consume more power and therefore generate more heat, so having more idle cycles keeps the heat down.)