The following is purely my opinion. Nothing below is warranted as true, trustworthy, knowledgeable or useful.
The way I look at it, the security of Airport is no better or worse than any other wireless router. The security is purely based on the level of security selected (none, WEP, WPA, WPA2) and the password used. Use WPA2 and a poor password and it's not much better than no security at all, it will just take more specialized software to get past the WPA2, once past that, a poor password is no problem.
Apple's Airport is not that much different than Linksys, etc., it's just a more user-friendly GUI and the Mac and the router can communicate with each other automagically without the user needing too many geek terms.
The security, in my opinion, comes from the fact that access to the file system can be set to block most snoopers, even without the wifi.