Johannes Schlüter has a new post for those wondering what's going to happen to PHP 5.3 and 5.4 now that PHP 5.5 has been released.

Yay, finally we released PHP 5.5, which is a new big release for PHP. In preparation for this I yesterday sent out a mail to the PHP core developers stating that the "PHP-5.3 BRANCH IS CLOSED NOW" [...] What this means for users of PHP is that they can continue using PHP 5.3 and when upgrades come they are very low risk of breaking anything (we always try not to break anything, but any person's bug might be another person's feature) so they should be applicable easily and applied fast.

So, PHP 5.3 users can upgrade in the 5.3.x series without having to worry too much about breakage. PHP 5.4 users should be able to easily upgrade to 5.5 without many issues too: "PHP 5.5 also mostly uses the same code as 5.4 with a few extra features."