In my experience, it's ridiculously easy. I installed an Airport Extreme card in my 12" iBook about 5 days ago, and it took me more time to find a coin to unlock my battery than it did to install the card itself. Unlock the battery, lift off the keyboard, slot the card in and connect the antenna, clip it down, put the keyboard back, put the battery back, switch on, and hey presto! Up and running wirelessly.
😀