The new kernel used in Intrepid seems to introduce a network bug. The problem occurs with the 32 and 64 bit version and is not limited to Ubuntu, I can reproduce the same bug with the latest OpenSUSE beta (11.1b3).
When booting from one of the LiveCDs or also from a fresh, clean installation I can not connect to any computer on my network or any internet page. I tried using the KDE4 network manager to set a static IP but I can't even ping myself on that IP. When I set an IP with "ifconfig eth0 IP_ADDRESS" I can access the network, but internet access still fails.
First I thought the bug was related to my onboard network chip (Realteak r8168) because there were some bug reports about the r8168/r8169 module, so I installed another network card (the chip on it says "Windbond W89C840F"). But I have the same problem with that card. I can even reproduce this problem on my two other computers.
My network setup is kind of uncommon: I'm using a "Speedport W501V" as a router, but I connect through a "Linksys WRT54G" with the DD-WRT firmware which I use as a WLAN bridge.
Older Ubuntu versions always worked fine for me, so I really think this bug is part of the new kernel.
If you need any further information I will provide it ASAP.
I have the same problem. With 3 of 4 PCs i could'nt connect to the LAN. The one that's working is an old Laptop with SIS Chipset.
I have this problems with a Billion Router BiPack 6xxx Router. Then i used a Broadband WLAN Router and it works