http://en.wikipedia.org/wiki/AirPort#Integrated_AirPort_Extreme_802.11a.2Fb.2Fg_and_.2Fn_cards said:
Integrated AirPort Extreme 802.11a/b/g and /n cards
As AirPort Extreme began to come standard on all notebook models, Apple phased out the user-installable designs in their notebooks, iMacs and Mac minis by mid 2005, moving to an integrated design. AirPort continued to be an option, either installed at purchase or later, on the Power Mac G5 and the Mac Pro.
With the introduction of the Intel-based MacBook Pro in January 2006, Apple began to use a standard PCI Express mini card. Cards with this form factor are now used in all AirPort-equipped Macintoshes.
In early 2007, Apple announced that most Intel Core 2 Duo-based Macs, which had been shipping since October 2006, already included AirPort Extreme cards compatible with the draft-802.11n specification. 802.11n capability was unlocked by an enabler included with the new draft-802.11n-capable AirPort Extreme Base Station, or by purchasing the enabler separately from the Apple Store online.[8] This card was also a PCI Express mini design, but used three antenna connectors in the notebooks and iMacs, in order to use a 2x3 MIMO antenna configuration. The cards in the Mac Pro and Apple TV have 2 antenna connectors and support a 2x2 configuration. To see which protocols your AirPort card supports, use the Network Utility application located in (Applications -> Utilities) and look at the model. Click here for a screenshot [9].
