I just got my WR810N(US) today. Flashed OpenWrt trunk and everything worked fine (haven't tested USB though).
The US version is very similar to the EU version. They have same CPU, RAM, and Flash, so the same openwrt firmware just works fine. It is a little bit smaller than the EU version, because it does not have the hardware switch for AP/3G/ROUTER (2 programmable GPIO in OpenWrt). FCC have some nice pictures of it. https://fccid.io/document.php?id=2892948
Since TP-Link now locks firmware on US version due to FCC regulations, flashing openwrt directly from the web interface didn't work. Then, I tried TP-Link's TFTP recovery mode (press reset while booting). I used Wireshark to listen for ARP, but the little box never ask for an IP address. I guess TP-Link just removed the TFTP recovery function in US version in order to make it harder to flash 3rd party firmwares.
Flashing via serial console should still work as always, but I didn't bother to try it. Thankfully, I found this method from our forum, which basically disables the firmware check via shell injection in the web interface. It did the trick and now I'm running openwrt. 
OpenWrt trunk for WR810n: https://downloads.openwrt.org/snapshots … actory.bin
Some picture of it (smaller one) with my old WR720n (larger one):
(Last edited by edwards on 22 May 2016, 08:11)