Many vendors today offer several battery options for more hours of operation, including additional batteries that fit in media bays, and higher-capacity main batteries. Another choice is third-party "battery slices" big enough to run your notebook possibly all day, if you don't mind the extra size and weight:
ElectroVaya's Power Pads currently come in three sizes, with the model number reflecting watt-hours (Wh): the PowerPad 80 (1.4 lbs., $249); the PowerPad 120 (1.9 lbs., $299); and the PowerPad 160 (2.4 lbs., $499).
Valence's N-Charge Power System uses a differentand, Valence claims, safer and more environmentally friendlylithium-ion chemistry than everybody else's options. The Valence lets you use your notebooks and simultaneously charge a PDA or cell phone. Valence has two N-Charge models, measured in Wh and Amp-hours (AH): the VNC-65 (5 AH, 6065 Wh, 1.8 lbs., $199) and the VNC-130 (10 AH, 120130 Wh, 3 lbs., $299). For comparison, the bay battery for my IBM ThinkPad is 8 oz., 2.2 Wh, $169.
My thoughts: Possibly a good solution, if you've got a long day away from a power socket, and aren't going to be walking around a lot.