I think I've finally found a USB-to-Ethernet adapter that works reliably with the Webplayer!
First, my configuration:
'Standard modem' webplayer, with 20GB IBM Travelstar DJSA-228 hard drive on the IDE port.
Linksys USB100M plugged into one of the USB ports.
Software is the Win98Lite/IE Doc.
After going through a ton of adapters (all the ones that Fry's stocks, including the D-Link USB Hub/Ethernet, the D-link 10MB Ethernet dongle, the 3COM 3C460B 10/100 USB Ethernet dongle, and the Hawking UF100 USB Network adapter), it looks like the Linksys USB100M is the one we've all been searching for.
After installation against the Win98Lite/IE image (which was painless), I can now see all the computers on my network. File navagation works correctly, and initial test with small fiels (~100K) looked good.
The I went for the 'biggie' - the big files test. I successfully transferred file sizes of 100M, 600M, and 1.7 GB (!!!) successfully to the webplayer from another machine on my network.
Currently, I'm transferring 600 or so MP3's from my MP3 server to the webplayer - It's been transferring music now for a couple of hours continiously without a hitch!
So, I think we have a winner!
If anyone needs any more info, just ask.