Depends on the HW and SW
I've played a little with this on a couple of different laptops and a couple of different distros. The answer really comes down to the hardware you are running and the built in drivers in the distro.
The first key thing I was found is that new laptops need the newest distros and drivers. The laptop is evolving really fast. Old laptops (>5 years old) may only have spotty hardware support.
For general use on an older laptop, Mandriva seems to have a bit of an edge. It has a longer history. If you are using a newer laptop (<2 years old), I would go for Ubuntu. It is slightly more user friendly than Mandriva (which was the one time most user friendly distro), and it is being kept very current.
To be more specific, I find that XUbuntu is a good choice for a lightweight environment for a laptop. It uses GTK, but does not have the extra weight of a full Gnome setup. A stripped version with just Firefox, Open Office, Thunderbird, Gimp, and a few other basic utilities will fit on a 2G flash drive (Should you need something to travel with). If MythTV, Scorched3d, bzflag, Freeciv, pioneers, googleearth, 500 megs of music, and a few other programs foor "in flight entertainment" are needed, you can still get it all on a 4G drive.
The key to battery life comes down to speed scaling. On one 2 year old Compaq nc6400 laptop, the SpeedStep technology of the Centrion Duo processors/chipset does not work in Ubuntu 8.04. (It also doesn't work in Win2K -- so it may be some a bios or hardware issue.) On a 1 year old Compaq 6910p with a Centrino Pro, Ubuntu seems to be able to throttle the CPU fine. All of the other built in hardware is well supported.
BTW, if FC5 is installed on the 6910p, the onboard LAN and WAN are not recognized. (FC5 had to be put on it to support some development tools for an embedded product.)