For our own hosting:
Our old servers still run PHP4/MySQL4, with PHP run as an apache module. We've never updated them to version5, and we probably never will.
New servers are being built with PHP5/MySQL5, with PHP run as CGI.
We give all our clients the option to move to new servers. As old servers empty we'll either repurpose them or retire them.
Servers for our dedicated server clients:
We offer whatever they want. When they ask for reocmmendations we recommend PHP5/MySQL5, CGI.
Jeff