Did you run your test with or without Jumbo Frame enabled?
Can you run iperf between the machines? I ran iperf last night on a kernel that had debugging enabled and it was running in the range of 200Mbits/sec. After installing a kernel with debugging disabled (2.6.30-rc4) it's running at 644Mbits/sec. An Athlon machine runs at 926 Mbits/sec.
644 is acceptable but 200 just didn't seem like it was enough. I tweeked a lot of ipv4 settings trying to get my speed up and now I realize the settings weren't a problem. It was just debugging slowing it down.
I did a timed copy of a 50M file to the sheeva and it did about 100MB sec, copying from the sheeva did about 20MB sec. This of course is to/from a Linux server. Scp was wayyy slower, the little sheeva just can't keep up with then encryption.