Been there, done that. My 2007 MBP is on its third battery, which is currently at the "Service battery" point.
If you don't discharge it deeply very often, make sure to calibrate it occasionally by letting it run through an entire charge-discharge-recharge cycle. This will improve your computer's estimate of your battery life, which has usually been a good thing in my experience. Here are Apple's instructions:
Calibrating your battery ensures you
get the longest possible running time
from it. Calibrate your battery during
the first week you own it, and then
recalibrate it every two months. If
you use your computer infrequently,
it’s best to recalibrate it at least
once a month.
Whenever you purchase new batteries,
calibrate them as well. To calibrate a
portable computer battery:
Plug in the MagSafe Power Adapter and fully charge the battery.
When the battery is fully charged, the light on the MagSafe Power Adapter
connector changes to green and the
Battery icon in the menu bar indicates
that the battery is charged.
Allow the battery to rest in the fully charged state for two hours or
You can use your computer during this time as long as the power adapter
is plugged in.
With the computer still on, disconnect the power adapter and
continue to use your computer.
When you see the low battery warning, save your work and close all
applications. Keep your computer
turned on until it goes to sleep.
After your computer goes to sleep, turn it off or allow it to sleep for
five hours or longer.
Connect the power adapter and leave it connected until the battery is
You can use your computer during this time.