Matthew Weier O'Phinney recently posted about a method he's come up with for migrating your object-oriented libraries (including frameworks) over to the upcoming PHP 5.3 version of the language.
With PHP 5.3 coming up on the horizon, I'm of course looking forward to using namespaces.
He gives an example of how useful these namespaces can be for you and your code, but points out one failing point - trying to define classes in a namespace that are named the same as a built-in keyword for PHP. There's already been a suggestion to add a captial "I" in front of the class name to prevent this collision.
There's also the problem of throwing custom exceptions - unless you use the namespace properly your script will just throw a default exception.