John,
I have not noticed a difference in battery life.
Here's a powerconsumption chart vs. a 5400 rpm, it uses .5v more on spinup and .2v less on writes so overall it's a wash.
# 60GB/5400 RPM vs 60GB/7200 RPM:
Max (startup/spinup) - 5W vs 5.5W
# Seek (avg): 2.6W for both
# Read (avg): 2.5W for both
# Write (avg): 2.7W vs 2.5W
(7K60 slightly lower)
# Performance idle (avg): 2.0W for both
# Active idle (avg): 1.3W for both
# Sleep: 0.1W for both
My 17 in originally came with an 80 gb drive and I opted for less space for better performance and you can always get an external usb case for the original drive and make it portable.
Something like this, you can find them USB powered or powered by an adaptor. I have also seen one or two firewire cases and there is one out there that runs on rechargeable batteries.
http://www.ramelectronics.net/html/NST-250U2.htm
The cases are only slightly larger than the drive itself.
scott