Rob "Pringles Can" Flickenger and others Cliff Skolnik at the O'Reilly OS X con has tracked down the cause of the annoying flakiness in the wireless network here — every 20 or 30 seconds, you start getting "connection refused" messages from your browser and other net-utilities. Rob "Pringles Can" Flickenger wrote it up.
It turns out that running the great network-spy app Etherpeg (or other "promiscuous" network sniffers) and the built-in firewall in OS X at the same time causes your computer to begin intercepting every packet sent out on your segment of the wireless network and respond to it with a "rejected" message.
So today, Rob (and everyone else who knows about this) is going to run around and tell people running Etherpeg to turn off the firewall (and vice-versa). Ah, fickle networking, you are such a stern mistress!
(Thanks, Rob!)