I saw a request for feedback, so here it is:
I have installed PHP 5.2.10 from the testing repo on 2 servers. One is a backup webserver that runs 4 Drupal sites. No issues with any of them so far, and I plan on upgrading the production server this coming weekend. The other is our mail server. It was the catalyst to try the upgrade, as our CEO insisted on getting Roundcube installed and it will not install with any version <5.2.
5.2's release was in Feb. 2008 and it seems to be mature and stable. It's been in Ubuntu since Hardy.
Thanks,
~J~