I haven't any (besides the standard kernel is not working) 
problems with network - my setup looks like:
lenovo 2522-3FG (thinkpad t410, i7/8G RAM)
kernel - kernel-xen-2.6.34.2-0.0.8.b3e335b.x86-64 
(kotd from 13.08.2010)
network - br0 on eth0, br1,br2,br3 on dummy0/1/2
I've tested a centos 5.5 pvm and windows 7 hvm, 
both have no problem with internet, 
both with 4 vif's, the first one connected directly to the br0 
(on eth0) , br1 and br2 are not connected to internet, 
br3 is connected via nat/masquerade and is forwarded to wlan0.
I never used network manager.
If you are interested in details of my setup, 
i can send you my config files
(domu, ifcfg scripts, firewall script).
Best regards
Maxim
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