Just ran into an error while installing Ubuntu 8.04 on a Xen patched Ubuntu server inside a VMWare virtual machine. Why would I want to do that you ask? So I can test the virtualisation of our file server and test application server.

The error occured after I tried to debootstrap a new ubuntu installation on my Xen installation:
cannot run command 'mount' exec format error

The solution: you are trying to install the wrong version, try the i386 or the amd64 version.
As simple as that.