After rebooting a DomU guest the configured pci passthrough card has vanished:

[root@tyche ~]# uname -a
Linux tyche 2.6.18-53.el5xen #1 SMP Wed Oct 10 17:06:12 EDT 2007 i686 athlon
i386 GNU/Linux
[root@tyche ~]# lspci
00:00.0 Communication controller: Cyclades Corporation Cyclades-Z above first
megabyte (rev 01)
[root@tyche ~]# reboot

Broadcast message from root (pts/1) (Mon Feb  4 16:15:24 2008):

The system is going down for reboot NOW!
[root@tyche ~]# rebootConnection to tyche closed by remote host.
Connection to tyche closed.
[athienem@localhost ~]$ ssh root@tyche
Last login: Mon Feb  4 16:13:14 2008 from nat-pool-str.redhat.com
[root@tyche ~]# lspci
[root@tyche ~]# 

shutting down the guest, waiting and starting it again makes the pci device work
again.
No messages about pci stuff in dmesg.