Perhaps my answer is not exactly what you have asked for, but it is widely used for prototyping.
Instead of a "real" 80c51 and an Ethernet Phy you may use a simple inexpensive FPGA board to configure your system the way you like.
There are many '51 cores available for free. Also there are good Ethernet cores ready to be integrated with '51 core.
All you need to do is put it together and provide an application binary.
This approach is much more flexible than a fixed hardware PCB and can be reused over and over again for different designs with other processors and peripherals.
Take a look at the
http://www.oregano.at/eng/index.html and **broken link removed** .
Those cores may be implemented on one of the boards available from this vendor :
http://www.digilentinc.com/ or from a hundred others.
I had had a more or less the same problem and decided to take an approach as described above.
Eversince I haven't used any "real" hardware processors or other digital chips any more.