Um - Lithium Ion batteries, which is what all recent and current Mac laptops use, do NOT experience the so-called "memory effect" seen in earlier battery technologies. Discharging them fully and recharging them completely only has to be done once (when you first acquire the machine) - after that it's a waste of time and battery power. (See wikipedia entry below - or any other authority on battery technology)
Resetting the PMU - instructions for Powerbooks and iBooks:
http://docs.info.apple.com/article.html?artnum=14449
How old is the iBook? Batteries don't last forever, unfortunately, and yours sounds very much like what I'm seeing with mine, in a 4-year-old TiBook. Fully charged and then boom - half an hour later almost nothing. 
It's worth reading the Wikipedia entry on Li-Ion batteries, and particularly this section entitled "Disadvantages":
QUOTE
A unique drawback of the Li-ion battery is that its life span is dependent upon aging from time of manufacturing (shelf life) regardless of whether it was charged, and not just on the number of charge/discharge cycles. This drawback is not widely publicized.
At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 degrees Celsius or 77 degrees Fahrenheit, will irreversibly lose approximately 20% capacity per year. However a battery stored inside a poorly ventilated laptop, may be subject to a prolonged exposure to much higher temperatures than 25 °C, which will significantly shorten its life. The capacity loss begins from the time the battery was manufactured, and occurs even when the battery is unused. Different storage temperatures produce different loss results: 6% loss at 0 °C/32 °F, 20% at 25 °C/77 °F, and 35% at 40 °C/104 °F. When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.
This makes Li-Ion batteries unsuitable for back-up applications compared to lead-acid batteries, and even to Ni-MH batteries.
Because the maximum power that can be continuously drawn from the battery depends on its capacity, in high-powered (relative to C, battery capacity in A·h) applications, like portable computers and video cameras, rather than showing a gradual shortening of the running time of the equipment, Li-Ion batteries may often just abruptly fail.
Given that my Ti is now entering its 5th year, I guess 30 minutes of battery power is to be expected!! Argh. Oh well - guess it's battery-shopping time soon.