- Go to "About This Mac" (top left under the Apple logo)
- Click "More Info..."
- Choose "AirPort" on the left-hand side (in the "Network" section)
- On the right-hand side look for "Card Type = AirPort Extreme"
- The "Firmware Version" underneath should indicate what chipset you have. E.g. "Broadcom BCM43xx"
Hopefully this will help you out. Maybe you want to know this because you're hoping to use Aircrack to do some WEP cracking :). This page lists compatible aircrack chipsets.