I use 5.3 across the board now. Generally, the latest stable version if it's on my own servers or whatever is available elsewhere like 5.3.3 on the shared server with Site5. From all accounts, that seems to be a pretty unusual situation to be in but I'm certainly not complaining. (:
Not in production, I run the latest developer snapshots (of the 5.3 and trunk branches) as often as I remember to rebuild PHP. I've also got a server which currently runs 20-something versions simultaneously... but that's another story entirely. :x
Because of the above, I tend to forget that most folks are still running 5.2 so very often suggest features which aren't available to them yet. As for PHP 6, how about moving that to a new thread? There's lots to be said but maybe not in this thread (hey, that rhymed).