Step 0 - Is the upgrade for you?Do you use InnoDB? This is where most of the improvements have been made, and it could be the main reason why you may want to be an early adopter. For a complete reference of the improvements check the New Features of InnoDB 1.1 section of the official MySQL Documentation, Sheeri Cabral's and Ronald Bradford's blogs.
Step 1 - Read the manualOk, I'm the first one to admit that I usually skip this step, but after a couple of false starts I decided that in this case it might be a good idea to go through the documentation, in particular Upgrading from MySQL 5.1 to 5.5. The Incompatible change entries are quite important in this release, and as you will find out, make the upgrade process a little more complex than usual. You mileage may vary, but make sure you're review them carefully.
Step 2 - Take a backupI usually trust (since we test them) the overnight backups, in this case, I'm a little more paranoid since you may not be able to do a straight upgrade and will need to remove the old version before you install the new one(see next step). Taking a new backup or double checking your last backup is a very good idea. Make sure you include your current configuration file(s) with the backup.
Step 3 - The upgrade processIf you have been using the InnoDB plugin, you need to disable the plugin options in your configuration file before you start the upgrade. The details are in the Upgrading from MySQL page I quoted in Step 1, it's the first Incompatible change.
Although it wasn't listed in the documentation, when tried to run an RPM upgrade (rpm -Uvh MySQL-server-5.5.7_rc-1.rhel5.x86_64.rpm) it failed with a message telling me that I needed to remove the previous version and run a regular install (rpm -i) instead. I usually get a little bit anxious with this kind of recommendations, but fortunately both steps ran smoothly.
Keep in mind that the first time you'll have to start the service with the --skip-grant option to run mysql_upgrade. If you're upgrading a production server, it highly recommended you add the --skip-networking otherwise you may get connections from your application(s), which you it's not a good idea until at this point. This is the 3rd Incompatible change. Once you're done, shutdown the MySQL service (ie: using mysqladmin shutdown, no need for username/password in this case) and restart it as you normally would (ie: sudo /etc/init.d/mysql start)
At this point your installation should be ready for testing. As I mentioned before, make sure you double check the Incompatible change notes to make sure your database configuration and/or application doesn't need any further adjustments. I would also recommend following closely the posts in Planet MySQL for reviews and articles from the community.