[iBook] Wireless network problem

[ibooklist at matthurst.fastmail.fm]

I should've known it. Having extolled the virtues of Macs just working
to a mate in the pub the other night, my iBook has decided to stop
logging into my wireless network. 

It finds it, but then reports 'there is a problem logging into
[network]. Try again?' I do, but get the same message over and over. My
PC uses the same router on an ethernet connection and is fine. A reboot
doesn't work.

I'm not sure where to even start looking for the problem. Any advice?

thanks in advance

Matt