If you have an eth0 device, the hardware is recognized and the driver is
loaded. Apparently the link is not ready.
*) Could the cable be faulty?
I'm afraid not, in fact this networking trouble only happens when I am within Xen, sorry I forgot to precise it. When I am running my 'basic' Lenny non-virtualized, network is fine, with the same cable. I think this answer is also good to go for your following questions :
*) To what switch is the NIC connected?
*) For gigabit, you need 8-wire cables, for 10/100 you only need 4 wires
(pins 1/2 and 3/6). Is that correct?
*) If connected to a switch, is STP active (port not yet in FWD, takes
up to 50s)
*) If your switch is manageable, what does that side 'say'?
*) Is there a speed/duplex mismatch?
No sign of such a message, so I guess no problems on that.
I really think the problem is Xen-related, as no problem at all are to be noticed with this connection when I boot up my basic system. But thank you for your input anyway, reminds me of my CCNA preparation :-)