I seem to have a similar setup to 'mjhorsnell' (Ubuntu Studio Jaunty).
When I execute "$ sudo dpkg-reconfigure udev" /sbin/udevadm will be deleted.
After doing "$ sudo aptitude reinstall udev" it shows up again.
So for me the fix was to run "aptitude reinstall udev" and then "update-initramfs -u -k all".
I seem to have a similar setup to 'mjhorsnell' (Ubuntu Studio Jaunty).
When I execute "$ sudo dpkg-reconfigure udev" /sbin/udevadm will be deleted.
After doing "$ sudo aptitude reinstall udev" it shows up again.
So for me the fix was to run "aptitude reinstall udev" and then "update-initramfs -u -k all".