Someone else in a previous thread pointed this out and helped solve my speed problems with my Asus RT-n16, and I thought it was important enough to bring out into the open in its own thread.
To achieve 802.11n speeds on your router, you MUST use WPA2 and AES encryption. I had mine set to WPA/WPA2 with AES-and-TKIP selected, and it was reverting to 802.11g speeds. Once I changed it to WPA2 and AES, suddenly things were much faster.
The reason for this is that WPA2 and AES are required as part of the 802.11n specs—it's written into the specifications.
So if you're having slowness issues with your 802.11n router (I thought I had bought a lemon!), the first thing to do is make sure you're using WPA2 and AES.
I'm not aware of any caveats to this, but if there are, feel free to add them to this thread.