Had same problem in etch and moved to GNU/Debian sid but did not help.

Checked log file from
/var/log/xen/qemu-dm.xxxx.log
and discovered the message:

/etc/xen/qemu-ifup: could not launch network script
Could not initialize device 'tap'

I made a:
# cd /etc/xen
# ln -s scripts/qemu-ifup qemu-ifup

Now I have a perfect "windows in a box" with network connection.

Other checks:
I start "windows" with
# xm create xxxxx.vhm

inside xxxxx.vhm I have
vif = [ 'type=ioemu, bridge=xenbr0' ]

inside /etc/xen/xend-config.sxp I set:
(network-script network-bridge)
(vif-script vif-bridge)

My system is ( from grub menu.lst)
kernel /xen-3.0-unstable-1-i386.gz
module /vmlinuz-2.6.18-1-xen-686 ...

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