Does my battery need to be replaced?
The number one question that people ask when they are looking at getting a battery replacement for their Apple MacBook Pro is "Do I really need to replace my battery?". When it comes to batteries you just can't put an arbitrary time cap on the replacement period as the life of Lithium-ion batteries is defined by the number of charge cycles the battery has already gone through. Apple does a stellar job of defining what a charge cycle is — A single charge cycle implies using and recharging 100% of battery capacity which doesn't necessarily need to happen in one sitting.
If you are running Snow Leopard then you'll be able to easily identify what state your battery is in by just clicking the battery icon or percentage indicator in the toolbar. All Apple batteries have a chip in them which reports charge cycles and current holding capacity which makes it easy to judge what state the battery is in at the moment. The battery indicator will let you know if your battery needs to be replaced.
For everyone running OS X.5 and below, you'll need to get yourself a copy of CoconutBattery or Watts which allow you to assess the state that your battery is in. Other than the number of charge cycles themselves, a common indicator of a problem with your MacBook Pro's battery is that the computer will shut down suddenly without going into sleep like it normally should. A dramatic decrease in battery backup time could help point to a problem with the battery as well. You can also check the state of your battery in the System Profiler which you can access just by typing that into the Spotlight box. The battery state can be found under the Power option in the sidebar.
Apple has comprehensive replacement and service policies. Apple will do the replacement for free in most cases. All you need to do is pick up the phone and call up Apple Care at 1-800-APL-CARE (1-800-275-2273).
However, if for some reason your battery replacement isn't covered in the warranty then the following is the latest list of prices for different regions as provided by Apple. The first price in this list is for the 17-inch Apple Unibody MacBook Pro and the second price after is for the 15-inch and 13-inch MacBook Pro models.
United States $179 pre taxes / $129 pre taxes
Canada CA$219 pre taxes / CA$159 pre taxes
Europe €179 inc. VAT / €139 inc. VAT
United Kingdom £139 inc. VAT / £99 inc. VAT
Australia A$299 inc GST / A$199 inc GST