I've been testing RFC2217 (serial-over-IP) using some RB411 boards, connected over an 850Mhz 3G network with a SW MC8705 to connect to, read, and remotely configure some industrial RS-232 hardware. We're connecting to the boards over a PPTP VPN, and using the popular (among MT users!) VSPE software to do the virtual COM port stuff.
For the most part, it works, but I have found it to be a bit unpredictable and spotty. Sometimes I need to reboot the 411 board. I have yet to properly reproduce the issues (and identify some conditions under which it misbehaves), but suffice to say it's not rock solid.
I think QoS may be an issue so that's one of the things I'll be investigating. It seems less reliable when our network is under heavy load (the devices are on a m2m specific plan and don't have a public IP, so I have to go through our VPN). Surprisingly, ping is really bad - around 500ms - and the jitter ain't too pretty either.
I'm hoping that other MT users who have any experience with Serial over IP using routerboards will be able to chime in with their experiences, and perhaps pass on some tips or commisserations!