hi malcolm.
network problems are usually system-wide, but did you try to create a new user to see if the issue is the same?
are you using any vpn software (to access your computer at work, for instance)? if that's the case, it might be interfering with bonjour. check the faq's and see if there are specific steps you need to go through to get bonjour back.
someone i know was using vpn software and he had the exact same symptoms you're having. we archived/installed, ran all system updates and again he could see his speakers in itunes, his base station in airport admin utility, his printer, etc. we then reinstalled the vpn software. everything was fine...until he made his first connection to his work computer. at that point he finally decided to read the vpn software FAQs and found the correct settings.
if you're not using vpn software, some other non-apple software is probably interfering. virus protection? 3rd party firewall? your system could be corrupted as well. an archive/install should fix it.
http://docs.info.apple.com/article.html?artnum=107120
i hope this helps.