In the two and a half weeks since we deployed PHP5 as an option for all clients, we've received only a handful of tickets regarding script compatibility with PHP5, all of which have been resolved at this time. Most of the issues that turned up involved needs/desires for a specific php5 module that we missed in the initial upgrade process.
As the upgrade appears to have gone smoothly thus far, The transition to using PHP5 as the default for all *.php files was performed this morning. At this time, all *.php scripts are now running under the PHP5 system across all servers.