I figured dual batteries meant there was an axtra module for a 2nd drive or battery. So the reason there aren't dual batteries on the 4750 is because there isn't an extra module so you would have to take out the cdrom drive which I woudn't have a problem with.
it may weigh twice as much as a centrino but it will kick the suckers ass in battery life. if you pay for a high capacity battery that is. for my digital camcorder i have a 3 cell battery which provides up 9 hours using the lcd screen! camcorder's are battery hogs so i find that rather impressive.