How to upgrade CentOS 5.6 to 6.0?

CentOS forum moderators seem to indicate that the only "supported" way is to do a reinstall:

http://www.centos.org/modules/newbb/viewtopic.php?topic_id=29670&forum=14

Re: centos 6: straight 'upgrade', or start-from-scratch new install?" "Yes, you're suspicions are correct - a new install in the only recommended/supported path."

However, this doesn't mean that the upgrade isn't possible, but it may be messy and will be unsupported.

I think, instead of looking for an upgrade, you should look for a fresh install, as upgrade option will be a tedious task to bring the machine to a stable state as lots of packages and their dependencies with their latest version upgrades need to be satisfied. If you have existing applications running on Centos, then the situation will be worse, and if you have additional cpan tools or other modules that are not a part of Centos, then it will really be a difficult task, so it will be fine, i think, if you go for a fresh install and you will save a lot of time.

It is possible doing:

yum upgrade --enablerepo=centosplus

But, as said in previous comments, it is not recommended and it can has problems. So a clean/reinstall method is the most safe way.