The PHP online manual has a whole section on migrating from PHP 4 to PHP 5. This also covers new features of PHP 5 that do not hinder any migration attempts, but may be an interesting read nevertheless. You can find the migration guide at http://www.php.net/manual/en/migration5.php.
Derick Rethans gave a talk on migrating from PHP 4 to PHP 5. You can find his slides online at http://www.derickrethans.nl/pres-breaking/talk.html and a PDF version (however, in lower quality) at http://www.derickrethans.nl/files/breaking.pdf.
Finally, Zend features in its PHP 5 Migration section a (growing) list of case studies at http://www.zend.com/php5/casestudies.php.