October 6 Onwards
Recent versions labelled RT-N have a new wireless driver which supports the E4200. This is commonly being referred to as the "N" driver. The driver was integrated into TomatoUSB by Tedddy Bear, who uploaded the code changes recently into the common tomato git repository. Subsequently we've all been making TEST, or BETA builds only - I don't think anyone has claimed these to be stable releases.
The driver is bigger, and it makes the compiles bigger - so isn't being used for "normal" builds. It is also rather experimental in nature and some things probably need to be tidied up / optimized. I expect Teddy Bear will get around to that when he has time.
Having said that, it has made it possible for people to use 5GHz on the E4200 which they couldn't before, and most people report no problems at all. Exactly how many do have problems we don't know, but there have been a great many downloads already. For all intents and purposes, you may consider it stable.
The "N" driver will also kind of work with the RT-N16, E3000, and so on, but it seems to be rather dubious practice, many issues have been reported although the choice exists. So PLEASE use the RT builds for these routers unless you need a bad headache :-)
If despite this, you DO use the RT-N builds on earlier routers, please don't post complaints about poor performance in the forums.