Re: Can you upgrade PHP version ?
I wouldn't look for a PHP upgrade until CentOS is upgraded. CentOS 6 was just released, and it has a new PHP version -- but an OS upgrade isn't something one does to a working complex system on a whim; compatibility with the load-leveling architecture, cPanel and so forth have to be tested (you never go with anything a vendor puts on a fact sheet without testing, unless you like running with scissors), and it's something that's far more likely to happen in response to a problem than to the "latest and greatest" being out there. As for modifying the current setup: there's more than enough weirdness involved trying to put together a massively-shared hosting environment without monkeying with the stuff that came in the box as well.
“Beware of bugs in the above code; I have only proved it correct, not tried it.” --Donald Knuth
"It was as if its architects were given a perfectly good hammer and gleefully replied, 'neat! With this hammer, we can build a tool that can pound in nails.'" -- Alex Papadimoulis (on TheDailyWTF.com)