(Partly copied and pasted from another post of mine.)
The only 802.11n chipsets I know of that work as an access point in FreeBSD are the ones supported by the ath driver (Atheros) and the mwl driver (Marvell 88W8363).
Some supported 802.11n chipsets I know of for Atheros are 5416 (802.11b/g/n) and 5418 (similar to 5416, but I think also supports 802.11a and 5 GHz 802.11n). 802.11n rate control is not yet implemented in the FreeBSD driver for Atheros chipsets, so only up to 802.11a/g speeds will work.
The Marvell 88W8363 is an 802.11n chipset with support for 802.11a/b/g/n, but the driver doesn't seem to support 802.11n rates yet and was just added in FreeBSD 8.0, so it is not possible to use the card in anything before pfSense 2.0 beta. I've only seen one model of card that I know uses it, a mini PCI card (though converters to regular PCI are available), and it only seems to be available on eBay from around 2 to 4 different sellers in China.