Comment 33 for bug 604335

$ lsb_release -sd
Ubuntu 11.04
$ ls -l /sys/block/vda/serial
-r--r--r-- 1 root root 4096 2011-06-16 13:22 /sys/block/vda/serial
$ sudo cat /sys/block/vda/serial
$

That file should contain a serial number but it doesn't.

Same thing on Fedora (by way of the dm-devel mailing list)

http://www.redhat.com/archives/dm-devel/2010-September/msg00095.html

Follow up message has the quote "The qemu on the host isn't new enough to handle the request."
http://www.redhat.com/archives/dm-devel/2010-September/msg00145.html