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Not sure if that's normal with a MacBook, but with my 12" PowerBook, it would vary between 97% to 99% while fully charged. Never once saw it at 100%.

Have you calibrated your battery yet?
 
Have you calibrated it? Even after doing that though, it may be slightly above or below 100%.

Edit: Also note that your Mac should know not to bother charging the battery if it's at about 95% or higher when you plug your computer in...this helps prevent overcharging of the battery.
 
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