As with most Apple devices the battery in the iPhone can’t be replaced by the user. There are actually a couple of options available, it can be sent to Apple to have it replaced by them or a you can find a 3rd party local repair shop that can replace it. After checking local directories, a cell phone repair shop specializing in Apple products was found here in Ottawa. The cost of a battery replacement for the 3G was $39 and takes only 30 minutes to complete. If you send your phone to Apple the cost is $89 plus $10.77 for shipping. In addition, the Apple support site says that all data will be lost during the battery replacement and therefore the phone should be backed up on iCloud or on iTunes. During the replacement of the battery locally, a backup was not necessary as no data is lost in the process. It is suspected, although it can’t be confirmed, that Apple doesn’t actually replace the battery in your phone but simply sends another phone as a replacement. In any event, after replacement, the battery life of the iPhone 3G returned to what it had been originally. You can go to my page on the iPhone 3G to get more details about its performance.
Given the cost of battery replacement, another option to consider would be just to purchase a new phone. With present promotions from Canadian carriers it may actually be cheaper to get an older model of iPhone (e.g. 3GS or 4) than to replace the battery. Depending on where you are in your contract’s life this may not be an option though.