Apple - Battery warning "Service Battery" goes away by itself
I believe this is fairly common behaviour. You may want to calibrate your MacBook Pro's battery to help Mac OS X better gauge its health.
I have a MacBook with an older battery. The battery is suffering and frequently shows the "Service Battery" message in the battery status menu. It too will alternate between asking for a service to being in good health.
Apple Portables: Calibrating your computer's battery for best performance
- Plug in the power adapter and fully charge your PowerBook's battery until the light ring or LED on the power adapter plug changes to green and the onscreen meter in the menu bar indicates that the battery is fully charged.
- Allow the battery to rest in the fully charged state for at least two hours. You may use your computer during this time as long as the adapter is plugged in.
- Disconnect the power adapter while the computer still on and start running the computer off battery power. You may use your computer during this time. When your battery gets low, the low battery warning dialog appears on the screen.
- At this point, save your work. Continue to use your computer; when the battery gets very low, the computer will automatically go to sleep.
- Turn off the computer or allow it to sleep for five hours or more.
- Connect the power adapter and leave it connected until the battery is fully charged again.