AFAIK there are no drivers for USB to ethernet (could be wrong, though.
VGR usually has all the latest news on USB drivers, so keep an eye on there).
The best method of networking A1200s is to get a Mediator, and use Voodoo and Realtek 8139 PCI cards (you have to have the voodoo to use the network card due to Elbox's DMA system using Voodoo RAM).
The only other viable solution really is to use standard PCMCIA ethernet cards (there are drivers for cnet or NE2000 compatible ones on aminet); although if your A1200 hasn't had the CC_RESET fix, you may have to remove and re-insert the network card after every reboot (which could get a bit annoying

.