My wife received her replacement battery yesterday! A mere 10 days after I entered the details on Apple's support website! So much for the 4 to 6 weeks!!!
The new battery is brand new (as noticed by the dulling of the iBook's white compared to the new battery) and is covered by a 90 day warranty as the iBook is now outside of it's warranty period.
The box included an addressed label to return the battery and a phone number to arrange collection.
I've scanned in the letter which accompanied and attached it below. Interestingly, Apple are making alternative shipping arrangements for those battery packs which have signs of overheating...
How painless can it be - Good Job Apple!
Apple's accompanying letter