Just to give some real-world perspective on this:
I'm tough on my batteries. I don't do any of the battery maintenance "tricks" like running them all the way down or not charging them when they're only partially depleted. I plug in when it's convenient and I run on batteries when it's convenient. Generally my batteries last about 2 years before they start to have difficulty holding a charge. When I can't get a good solid hour and a half of real-world battery use, I buy a new battery. For me, $100 every couple of years is worth not sweating if I'm going to wear out my battery whenever I plug/unplug my iBook. Generally after 4 years I'm going to replace the computer anyway, so I've only ever replaced the battery on a given laptop once.