Warning
This is a job best carried out by someone who has some related experience, opening your iBook will probably void any remaining warranty. Bearing this in mind, my own experience with it has led me to conclude that anyone with a steady hand, reasonable eyesight, the ability to read and follow instructions and a decent level of patience would do this in a few hours.
Introduction
About a month ago Dawn's iBook started crashing for no apparent reason and we backed up all her data and tried diagnosing the problem using Techtool Pro, Disk Utility and DiskWarrior... no errors were returned or the applications locked up while checking. It was one of those classic situations when nothing seems repeatable so, after exhausting all other avenues we concluded that the most likely cause was a failing hard drive. We had recently purchased a couple of Iomega 80gb bus-powered firewire drives so I set up one as a mirror of Dawn's drive and for the last fortnight Dawn has been using her iBook booting exclusively from this external drive. This didn't settle the issue of cause as an unhealthy fan-like noise persisted even when the internal drive had been dismounted causing us to wonder whether the whole issue was an overheating one caused by a fading fan. Deciding the only approach was to deal with it one step at a time, I bought a replacement drive from www.Macupgrades.co.uk (IBM (Hitachi) Travelstar 2.5" IDE 5K100 80.0GB (5400RPM) (48) = £54.58 ex VAT). I also needed a T8 Torx screwdriver which I initially ordered from the same company but which I had to subsequently reorder from www.Mackay.co.uk as Macupgrades sent the wrong type of screwdrivers!
Procedure
Rather than going into details on how I did the whole thing I'll just refer you to a great site which provided a very detailed guide to the procedure, www.iFixit.com. I was able to download a PDF file which covered every step and showed detailed photos as well. I did try buying some extras from the site as an acknowledgment but was put off by a $35 shipping charge for $10 worth of goods. I used to get a load of stuff mail order from the States but they have since crippled their postal service and do it all by UPS nowadays... and it always seems to work out prohibitively expensive. To put this in context, the postage for my Hard Drive order from Macupgrades was £5.88, and this included a keyboard and two sets of screwdrivers.
Conclusion (I hope!)
The procedure went smoothly and the entire process took about three hours. The new hard drive seems to be doing what's expected of it and Dawn has been trying to stretch it to see if it gives any problems. In my opinion, the procedure was simple enough that I would now have no hesitation replacing a drive in any iBook or PowerBook. I would note, however, that more recent, faster drives might produce more heat than the existing drive and this should be checked carefully. The drive I installed runs at 5400rpm instead of 4200rpm but my research showed that it produces slightly less heat due to bearing improvements. If any problems manifest themselves I'll post them here.
This is the corner of our bedroom where I carried out the operation, the PowerBook on the right is displaying the PDF detailing the procedure (this could just as easily have been printed out). My biggest worry was the possibility of dropping a screw and losing it in the mess.