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.