Good question.

ANCIENT HISTORY:
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I investigated the feasibility (though I questioned the advisability) of putting TCP/IP capability into an existing creditcard validation terminal a few years back.

The company had a Zilog Z180 in the terminal and they were planning to buy a TCP/IP interface from a company in Scotland. The software required about 128K of code space. Luckily, I only had to show them that they had the hardware capability and that ended my obligations to them. Some poor guys in California had to do the software conversion.

THAT SOUNDS LIKE TOO MUCH CODE SPACE
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128K? I find it hard to believe it would take that much to do a simple device but it should at least make it clear that it is probably not a trivial thing to do.

TODAY WE HAVE CHIPS FOR EVERYTHING HARD
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And as you said, the chip-ports are available nowadays and that should make it easier.

WHAT'S WRONG WITH THIS PICTURE?
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But from a systems design perspective, I'd really doubt the advisability of putting a web of 8051s directly and individually on an ethernet. The whole advantage of 8051s is their low cost and simplicity. It seems wiser to me to connect the 8051s together via an RS485 LAN. If you need a portal to ethernet, tie one (or two for redundancy) from the array into a PC and let the PC handle the ethernet traffic.

DON'T CHROME A FORD PINTO OR CHEVY VEGA
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My guess is that the 8051 would be a poor candidate for TCP/IP unless you have a thoughtful and knowledgable document suggesting a MINIMAL set of TCP/IP protocol for LITE devices. I've not seen this document but I haven't looked either. I found a document for another notorious standard but that's another topic.

-Jay C. Box