I know that Network Magic has an "Intruder Alert" function, but it doesn't appear to be very good about popping up an alert and there is no email alert function. I did confirm that the "Intruder Alert" feature is active in the free version of Network Magic, though.
Another alternative might be AirSnare, which is also free and relies on using Ethereal / Wireshark.
What would be really nice would be if someone carried on the work done by this team from Georgia Tech described in this 2003 paper (PDF link). The team hacked a USR2450 11b access point to create a wireless intrusion and response system that looked for NetStumbler activity and launched a DoS attack against it in return.
Let me know about other solutions and I'll get this together into an article. Just open up the Comment box below and fill it in. Thanks!