I'm looking to build a pfSense router/firewall in the very near future and I'm hoping to use my Asus RT-N56U as the wireless access point - is it possible? Is it compatible?
Yes, that should be possible. Usually what you do is to turn off firewall etc., turn off NAT and DHCP, configure the wireless settings as desired then just plug the link from the pfsense-box into LAN port 1 instead of WAN port on the router.
This should be the general configuration on most home-routers.
EDIT: obviously it was quite easy.
"The final mode is the Access Point. This puts the RT-N56U in place as a simple wireless device with no firewall, NAT or other services. It will serve up both 5GHz and 2.4GHz wireless to your home and that is it."http://www.tweaktown.com/reviews/3853/asus_rt_n56u_dual_band_gigabit_wireless_n_router_review/index5.html