All of the things the Airport Express needs to do are already being done by the Apple TV, so the capability is there. The only sticking point is perhaps that 2

All of the things the Airport Express needs to do are already being done by the Apple TV, so the capability is there. The only sticking point is perhaps that 2nd ethernet port, the Apple TV with the A4/A5 seem to use a separate ethernet chip made by SMSC connected to the processor via USB. As far as I know they don't support ethernet internally but they may not need to either.