Painless! I’m actually starting to expect it to work without hitches now.
There are a couple of config file changes that need babysitting but none of them was difficult; I really do wish it would automatically do a three way merge between its old package version, the new version, and my version, and just assume “yes” if they merge cleanly.
Instructions are trivial: see Hardy Upgrades: Network Upgrade for Ubuntu Servers (Recommended).
This also works fine on Xen.