You can fake module naming with underscores (ugly but it works) and with duct typing you're only typing the full names of classes sparingly. In Java, the lack of namespaces would drive you insane, in PHP not so much. The lack of namespaces hasn't slowed down the development of very large class hierarchies in PHP -- just look at the Zend framework.

I'm not arguing against namespaces, I'm just saying for PHP they're more of a luxury than a necessity.