This link says that Magento is compatible with PHP 5.2.13 - 5.3.24, but when you run the PHP script to check server requirements, it says that is okay to run on 5.4 and even 5.5. But I've seen some issues with 5.4 over the internet.
So the question is: with PHP 5.3 reaching its End of Life, is it possible to run Magento on PHP 5.5 and have a server with a PHP version that lasts longer?
Yes it is, at least large parts of the code (as for any PHP 5.3 application). And as the source is available and rights to modify it are given to every user, for the parts of the code that is not compatible it is possible to remove any incompatibilities making it completely compatible with PHP 5.5.
If you need a more authorative statement, you need to contact the support, here is the sales funnel page: Magento Support Overview.