Your netcat test looks broken. As the guruplug uses the same phy for its network interfaces as the sheevaplug.
please try this
user@guru: nc -l -p 2000 >/dev/null
user@host: dd if=/dev/zero bs=4096 count=200000|nc guru 2000
819200000 bytes (819 MB) copied, 10.0561 s, 81.5 MB/s = 652Mbit/sec