I spent some time on mainline 2.6.33-rc8, which mostly worked well, until I plugged it into a Procurve switch. I appeared to be unable to send any packets while plugged into that switch.
I then found the kernels linked from this forum, and tried 18.104.22.168. At the same time, I set arcNumber to 2097. This combination worked great.
Looking through the kernel.org changelog and browsing the patch listing on the plugcomputer kernel download site, I can't see any changes that would affect the ethernet driver. Do you know where the fix might have come from? Was it just a matter of setting the arcNumber? Are the patches in the 'patches' directory of your download site the full set of patches applied to mainline?
PS: any chance of getting OpenID support for this forum?