Get out your configure scripts... php 5.2 was released last night. There's a new extension for creating zip files, hooks for tracking file upload progress and a few other niceties - like the JSON PECL is included by default.
I like the new upload features. That can really add some dimension to php applications however it's a little disappointing that many hosts have not upgraded.
A lot of hosts ran good old 4.x, no word about v5.x ... it takes a while to get in touch with a really good host
One thing I think that will hold back mass acceptance is that the Zend Optomizer isn't yet 5.2 compatible. I'm sure they'll release a new version before too long. I was pretty reluctant to upgrade to 5.x for a while myself, but I don't use a lot of code that needed upgrading.