I set the router up for WEP, simple to do, and simple to access from my PC (the config page gives you some generated keys, you write them down and type it in at the relevant point). The iBook fails to connect saying "There was an error connecting to $network_name". Which is nice and detailed. Trying to set up the connection by a different means, it tells me that the password is wrong. Despite it being the same as I've just put into my PC. After trying in capitals, and using the passphrase used to generate the password, I eventually give up and switch off the protection. I'm sure it's possible to do it (after all, the connection dialogue did quote "WEP", but I'm disappointed that it didn't Just Work after the PC doing it painlessly.
update: As well as Merlyn's advice below, blech pointed out a relevant apple document.