DomU isn't talking to the world, but it talks to Dom0.

Here are the tests that I made:

Dom0 (external networking is working):

ping 188.40.96.238 #Which is Domu's ip

PING 188.40.96.238 (188.40.96.238) 56(84) bytes of data.
64 bytes from 188.40.96.238: icmp_seq=1 ttl=64 time=0.092 ms

DomU:

ping 188.40.96.215 #Which is Dom0's ip

PING 188.40.96.215 (188.40.96.215) 56(84) bytes of data.
64 bytes from 188.40.96.215: icmp_seq=1 ttl=64 time=0.045 ms

ping 188.40.96.193 #Which is the gateway - fail
PING 188.40.96.193 (188.40.96.193) 56(84) bytes of data.
^C
--- 188.40.96.193 ping statistics ---
2 packets transmitted, 0 received, 100% packet loss, time 1013ms

The system is debian lenny with a normal setup.

Here is my configs:

uname -a

Linux green0 2.6.26-2-xen-686 #1 SMP Wed Aug 19 08:47:57 UTC 2009 i686 GNU/Linux

cat /etc/xen/green1.cfg |grep -v '#'

kernel      = '/boot/vmlinuz-2.6.26-2-xen-686'
ramdisk     = '/boot/initrd.img-2.6.26-2-xen-686'
memory      = '2000'

root        = '/dev/xvda2 ro'
disk        = [
                  'file:/home/xen/domains/green1/swap.img,xvda1,w',
                  'file:/home/xen/domains/green1/disk.img,xvda2,w',
              ]


name        = 'green1'

vif         = [ 'ip=188.40.96.238,mac=00:16:3E:1F:C4:CC' ]

on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'

ifconfig

eth0      Link encap:Ethernet  HWaddr 00:24:21:ef:2f:86  
          inet addr:188.40.96.215  Bcast:188.40.96.255  Mask:255.255.255.192
          inet6 addr: fe80::224:21ff:feef:2f86/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3296 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2204 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:262717 (256.5 KiB)  TX bytes:330465 (322.7 KiB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

peth0     Link encap:Ethernet  HWaddr 00:24:21:ef:2f:86  
          inet6 addr: fe80::224:21ff:feef:2f86/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:3407 errors:0 dropped:657431448 overruns:0 frame:0
          TX packets:2291 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:319941 (312.4 KiB)  TX bytes:338423 (330.4 KiB)
          Interrupt:16 Base address:0x8000 

vif2.0    Link encap:Ethernet  HWaddr fe:ff:ff:ff:ff:ff  
          inet6 addr: fe80::fcff:ffff:feff:ffff/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:27 errors:0 dropped:0 overruns:0 frame:0
          TX packets:151 errors:0 dropped:33 overruns:0 carrier:0
          collisions:0 txqueuelen:32 
          RX bytes:1164 (1.1 KiB)  TX bytes:20974 (20.4 KiB)

ip a s

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: peth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
    link/ether 00:24:21:ef:2f:86 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::224:21ff:feef:2f86/64 scope link 
       valid_lft forever preferred_lft forever
4: vif0.0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
5: veth0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
6: vif0.1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
7: veth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
8: vif0.2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
9: veth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
10: vif0.3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
11: veth3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN 
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
12: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN 
    link/ether 00:24:21:ef:2f:86 brd ff:ff:ff:ff:ff:ff
    inet 188.40.96.215/26 brd 188.40.96.255 scope global eth0
    inet6 fe80::224:21ff:feef:2f86/64 scope link 
       valid_lft forever preferred_lft forever
14: vif2.0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 32
    link/ether fe:ff:ff:ff:ff:ff brd ff:ff:ff:ff:ff:ff
    inet6 fe80::fcff:ffff:feff:ffff/64 scope link 
       valid_lft forever preferred_lft forever

brctl show

bridge name bridge id		STP enabled	interfaces
eth0    	8000.002421ef2f86	no		peth0
    						vif2.0

ip r l

Dom0:

188.40.96.192/26 dev eth0  proto kernel  scope link  src 188.40.96.215
default via 188.40.96.193 dev eth0

DomU:

188.40.96.192/26 dev eth0  proto kernel  scope link  src 188.40.96.238
default via 188.40.96.193 dev eth0