At the time of writing, the latest version of Joomla - 1.5.15 - is not compatible with PHP v5.3. Most web servers will remain on 5.2/5.1 for some time, however if you use XAMPP you must install 1.7.1 otherwise large portions of built-in Joomla functions will will break.
With that said, for obvious reasons we cannot support PHP 5.3 servers at least until a new Joomla version is released to address it. Thanks!
