Steve,
This is a very common problem.
There's heaps to try:
Calibrate battery (as James said):
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh2339.html
Battery Firmware update:
http://support.apple.com/downloads/Battery_Update_1_2  (depending on
your model)
See if you qualify for new battery:
http://www.apple.com/au/support/macbook_macbookpro/batteryupdate/
Check battery for "bulging" - Read this WAMUG thread from 29 Jan 09:
MacBook Pro Battery Warning (finished one)
This list is probably not exhaustive. Good luck.
cheers
Travis

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