This won't affect all of our customers, but we felt that it was prudent to share with everyone some information recently relayed to us.
To make a long story short, if you run Windows + IIS, we recommend that you do not upgrade to PHP 5.3. ionCube has released Windows PHP 5.3 loaders, however they will not support PHP builds that are built using VC9 compiler (which is what is used for the current releases on php.net for IIS). They expect to release an update sometime in first quarter of this year.
See this topic for more information: http://forum.ioncube.com/viewtopic.php?t=3094
Thank you to OutsideOfDreams for pointing this out.
(Note that if you are not using Windows, this does not affect you, you can run whichever PHP version you like)