I use also FC5 as Dom0 and have the same effect. I researched and found some 
more install-inforation here: 

http://www.netbsd.org/de/Ports/xen/howto.html#unprivileged-domains 

Here, they say, that it is important to set a root device, if you look the 
chapter "using Linux as a guest".

I imagine, it would be the same and that i would have to enter a root-device 
for netbsd.

that is what i imagine:

disk = [ 'file:/etc/xen/[yourimagehere],0x301,w' ]
root = "/dev/wd0d ro"

perhaps i am wrong, as i never looked in partition-scheme from netbsd, but 
perhaps a netbsd-guru can help us here ;-)

Roger