hmmm, very interesting! I knew the network adapter was a "potential" thing, didn't realise it was actually out in the wild so to speak 
just doing a quick search dug up some you tube videos. looks like this thing can run at about 90kB/s, so just under a megabit. certainly better than serial or parallel; and i'm hazarding a guess those video's were on older cores? but i'm purely speculating here.
i'll have hunt around and see if I can sort out a couple of adapters. would be interesting to side-by-side USB vs SD network connections
If you want to go the SDnet route I can get you in touch with the builder of them. They are very simple to use, and honestly the SD card slot is worthless for using an SD card in. Its very buggy. A large hard CF or SSD on the IDE port is much better.
As far as the A500 clockport adapters and rapidroad I remember an issue being reported with it. I will look for the thread, but I do remeber I quite looking for this adapter once I saw that.
