I'm about to run a release upgrade from Jaunty to the beta of Karmic. Every time I have done an Ubuntu release upgrade in the past there have been at least some issues. My goal is to take all of the wisdom and experience I have from these past attempts to ensure that everything goes smoothly with this upgrade.
Helpful tips for release upgrades
The do-release-upgrade command is a tool created by the Ubuntu developers to make moving between release versions easier. This tool takes care of updating all of your package sources, updating your packages, and even has a couple of features designed to prevent massive system inconsistency (which is the typically worst thing that can happen during an upgrade), such as an automatic rollback of your sources if something goes wrong in the early stages.
The screen command is a tool which allows you to run a shell that can be dynamically connected and disconnected from interfaces such as running konqueror locally, or when logging in remotely using ssh. Using this tool can be extremely helpful for those cases that the release upgrade breaks your running X windows environment, or some type of keyboard interface. You can pick up exactly where you left off in your upgrade from a remote machine (assuming SSHD is still running), and hopefully come out less scathed than if you had to force reboot, or kill the upgrade process in the middle.
Back up your /etc/X11/xorg.conf
The package upgrade process is already going to make a backup of this file if it makes any changes, but an extra backup can't hurt, and can actually make it easier to recover. It ends up being easier to recover if you know exactly what you had working before the upgrade. There are commands like sudo dpkg-reconfigure -phigh xserver-xorg that will reset your X server configuration, but this isn't always desirable if you have changes such as proprietary driver configuration, meta-modes setup for multiple monitors, or other custom changes.
Make sure your RAID array is stable
Have a quick look cat /proc/mdstat and make sure any RAID arrays you have are healthy. You are asking for issues if you have a degraded array when you start a major upgrade.
Ensure any encrypted filesystems are healthy and you know how to recover them
I don't have personal experience with Ubuntu-managed encrypted filesystems, but OS-managed services like this seem to be one of the major things that have historically had problems with release upgrades.
This item won't affect how well your update proceeds, but it will be something you will need to do now if there is any chance you will want it later. Often after an upgrade, things will seem slower or faster. The only way to be certain is to take measurements. You can use tools, such as bootchart, or just use a timer to manually check things to get a sense of how things are changes. 9.10 and 10.04 are supposed to be bringing some major speed improvements, so make sure you take some measurements before and after the upgrade to see how things are coming along, and to see how the updates affect your environment.