For the sake of completness, it's not a hardware issue. The system can be netbooted via PXE just fine and both on-board interfaces have link:
root@grml ~ # mii-tool -v eth1
eth1: negotiated 100baseTx-HD, link ok
product info: vendor 00:08:18, model 22 rev 2
basic mode: autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
link partner: 100baseTx-HD 10baseT-HD
root@grml ~ # mii-tool -v eth2
eth2: negotiated 100baseTx-HD, link ok
product info: vendor 00:08:18, model 22 rev 2
basic mode: autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
link partner: 100baseTx-HD 10baseT-HD
I guess this calls for a reinstall of ESX then :-\