The Pi would be infititely more useful to me if the ethernet interface was attached to a "real" bus and not hung on USB. Here's to hoping that will be the next upgrade!
The only interface that can do DMA is the USB one. All the others have relatively small FIFO buffers and require the CPU to do all the I/O work. So you could attach an ethernet interface to one of the SPI buses, or bit bang a few GPIOs but surely the CPU load is going to be painful?
Edit: Oh wait, there's a main SPI bus that can do DMA as well. See page 148 on the doc linked above. If you can find a suitable chipset & driver then have at it :)
(That datasheet is also very incomplete - there's lots of other interesting stuff in the SoC like the MIPI CSI/DSI interface, and if you felt perverse enough and had the right skillset you might be able to get Ethernet-like data transfer out of one of those...)
Your home network is likely already faster than the Pi can handle, let alone your internet speed. A file transfer across your network (scp a video to the SD cart for example) will already burst, then slow down as-is. You can see, the SD cart write i/o is the current bottleneck.