Not to mention that in addition to probably not having a backdoor, it is also way more stable than the junk firmware that a lot of routers have. I had a linksys that would need to be restarted every few days to clear out the cache of the router because it stopped working. Installing the the open source firmware made it rock solid and added a bunch of features that you'd only get in expensive routers.
Wireless routers typically have just one NOR flash chip as the only nonvolatile storage device, and any firmware needed by the WiFi devices is uploaded when the devices are initialized by the OS. There just aren't many places to hide something like a rootkit, and that's part of why these backdoors are so easy to find by inspection.
What you would have to do is find out which routers have more "long term" support. Those are the ones that tend to have the better developed software and less security holes. Those also tend to be the more expensive ones for that exact reason. Once you bought that router, they are getting no more money from you, so if they want to keep it supported, they have to charge more to make up for the cost of software support.
