We are evaluating Lantronix (E.g. XPort) and Tibbo (can't recall product name). They are a bit pricey (approx $40 Q1) but are pretty easy to use and full-featured. It would take a lot of work to have such a polished equivalent to sell to your customers (if that is your intention). If memory serves, they both support telnet, ftp, http, probably email, SNMP... and don't forget Virtual Serial Ports.

I think both of these companies also allow you to develop on THEIR embedded micro so you can do away with your 8051 if you want. I think they are almost like having an '186 (PC AT) in a thumbnail size. You would be pin constrained though.

Other option might be SitePlayer. It's 8051 but more limited.

GB