Currently we are testing PHP 5.3 on some of our production servers. It has a quite stable feel to it.
I expected much more problems. Although we had to modify our sources a bit, (To avoid this bug for example. Using empty() within our own database class extending mysqli) the effort into getting it to work wasn't too big.
In the past years we've always used the dotdeb.org repository for php packages. Guillaume is doing a terrific job in keeping his packages updated with the last PHP source version. Typically it takes about 2-3 days for packages tot pop up on dotdeb after a new PHP release. Because 5.3 is such a big change, Guillaume has setup a separate repository for it.
The dotdeb php 5.3 repository is still lacking some packages, so we had to build a few packages ourselves. The packages php5-apc, php5-ffmpeg and php5-mailparse built the dh-make-pecl way without problems.
Building php5-imagick gave us some troubles. I don't recall if that was because we wanted them running on ubuntu too. The ImageMagick package is in transition from libimagick to -core and -wand packages. Also debian and ubuntu packages names are different. I eventually decided to build ImageMagick ourselves too. That way I have the same setup on production and my ubuntu development laptop.
We are using the syck pecl package, that one built as a charm too. We use the libsyck0-dev package from debian unstable to build it.
We also used the dbase functions that were available by default in PHP 5.2. From 5.3 on they have been moved to a pecl package, but it is still without maintainer. Currently it lives in SVN only at http://svn.php.net/repository/pecl/dbase/. This one wasn't easy to build. You have to make a file structure that complies with pecl packages, before you can use dh-make-pecl on it. If you want I could provide you a debian source package for this one.
Now the only thing missing is the Suhosin patch for PHP 5.3. Good news on that front: Arnold Daniels ported the patch http://blog.adaniels.nl/articles/suhosin-patch-for-php-53/. Although it is not done by Stefan Esser of the Suhosin team himself.
Did you switch to PHP 5.3 already? Did you run into any problems?