Jon,
Actually, SME 5.1.2 ships with PHP 4.0.4pl1. This upgrades it to 4.0.6, which still isn't new enough to run IMP 3.
Everyone else,
No, I can't help you at the moment, as I haven't been able to get the update blade installed. I expect, though, that you'll have one of two options if you want to use IMP 3: (1) do NOT install Update 1, or (2) install the PHP RPMs with --nodeps.
I've built RPMs of PHP 4.1.2 which incorporate Mitel's patch, but they still (probably) won't play well with Update 1.