> What domU kernel are you using? Does it have xen-pcifront? 
I first used not a own domU kernel (dom0 Kernel: 2.6.32-5-xen-amd64). This did not wirk, the netowrk came up and got down immediately:
[  260.736409] e1000: eth1 NIC Link is Up 100 Mbps Full Duplex, Flow Control: RX/TX
DHCPDISCOVER on eth1 to 255.255.255.255 port 67 interval 5
[  261.552116] e1000: eth1 NIC Link is Down
[  264.768408] e1000: eth1 NIC Link is Up 100 Mbps Full
 Duplex, Flow Control: RX/TX
[  265.580116] e1000: eth1 NIC Link is Down
DHCPDISCOVER on eth1 to 255.255.255.255 port 67 interval 8
[  267.200418] e1000: eth1 NIC Link is Up 100 Mbps Full Duplex, Flow Control: RX/TX
[  268.012111] e1000: eth1 NIC Link is Down
[  269.632409] e1000: eth1 NIC Link is Up 100 Mbps Full Duplex, Flow Control: RX/TX
[  270.444115] e1000: eth1 NIC Link is Down
I then followd the instructions on http://wiki.debian.org/PyGrub and used the 2.6.18 kernel, but this did not change anything.> What does "lspci" show in the domU?
domU:~# lspci
00:00.0 Ethernet controller: Intel Corporation 82541PI Gigabit Ethernet Controller (rev 05)
> What pci related do you have in the domU dmesg? 
domU:~# dmesg | grep -i pci
[    0.000000] Allocating PCI resources starting at
 8000000 (gap: 8000000:f8000000)
[    0.013875] PCI: setting up Xen PCI frontend stub
[    0.185563] pcifront pci-0: Installing PCI frontend
[    0.185826] pcifront pci-0: Creating PCI Frontend Bus 0000:00
[    0.189811] pci 0000:00:00.0: reg 10 32bit mmio: [0xfeac0000-0xfeadffff]
[    0.189811] pci 0000:00:00.0: reg 14 32bit mmio: [0xfeaa0000-0xfeabffff]
[    0.189811] pci 0000:00:00.0: reg 18 io port: [0xb400-0xb43f]
[    0.189811] pci 0000:00:00.0: reg 30 32bit mmio pref: [0xfea80000-0xfea9ffff]
[    0.189811] pcifront pci-0: claiming resource 0000:00:00.0/0
[    0.189811] pcifront pci-0: claiming resource 0000:00:00.0/1
[    0.189811] pcifront pci-0: claiming resource 0000:00:00.0/2
[    0.189811] pcifront pci-0: claiming resource
 0000:00:00.0/6
[    0.234504] sysfs: cannot create duplicate filename '/devices/pci-0/pci0000:00/0000:00:00.0/config'
[    0.236132]  [<ffffffff811a5303>] ? pci_create_sysfs_dev_files+0x46/0x356
[    0.236132]  [<ffffffff8119e702>] ? pci_bus_add_device+0x32/0x39
[    0.236132]  [<ffffffff8119e733>] ? pci_bus_add_devices+0x2a/0x11e
[    0.236132]  [<ffffffff812fbdc1>] ? pcifront_scan_root+0x19f/0x1c4
[    0.236132]  [<ffffffff812f8e84>] ? pcifront_backend_changed+0x176/0x4e3
[    0.732604] e1000 0000:00:00.0: Xen PCI enabling IRQ: 17
[    0.998732] e1000: 0000:00:00.0: e1000_probe: (PCI:33MHz:32-bit) 00:0e:0c:b9:9b:4a