At ZendCon I talked about "Planning a PHP 4 to PHP 5 codebase rewrite, a practical approach". The talk was based on my own experience, as well as famous discussion of the topic such as Joel Spolsky's "Things you should never do" and the examination of "famous" platform rewrites.

Aaron gives an example of a large company making a move from a COBOL system out to C for a mission critical system. Based on his tale, they didn't put the thought needed into making this move - new development time, keeping old developers on staff, etc - besides the fact that customers don't like change and making a move to another platform is almost definitely going to be noticed by them.