Brandon Savage has posted his own response to the "don't be afraid of the upgrade" thread that's been going around concerning updating to PHP 5.3. He takes a different approach to the matter, though - one of a bit more caution.
When it comes to open source projects endorsing PHP 5.3 as their one and only PHP platform, I encourage caution. When it comes to open source projects that use PHP, there are three main issues that I believe should be considered before making the leap to PHP 5.3 (or any new release of any new software).
He sees three considerations projects and companies should consider before making the upgrade on their servers - the amount of control you have over the environment, deciding if the upgrade is worth possible backwards compatibility breaks and how much support you want to still provide for the pre-5.3 version(s).