It's in the network settings. If the machine is set to "Automatic" then Airport won't connect until a user logs in. If you specify a network, the machine will connect on its own at the login window. Spiffy, eh? It took me a loooooong time before I stumbled across this one. One day I just tried it and all my problems were gone.
...you know, spontaneously my dog came back, my truck started working, my wife came back, the creditors stopped calling, my bank account became full, and oh yeah, I could turn off the Unix scripts I had written to fix the DNS issues and hence Kerberos and Open Directory issues that were created by there not being a network port available on startup.