I have a beta script (xenTest.php) ready that is just code, no pretty page, that goes through a list of things that might be required, and reports back with an Ok, or list of feedback.
If XF does not come with one, I will publish it on xenfans site when the time is right. It's too early in the development cycle to know yet (set in stone wise) what XF requires.
At the moment I would say that if you run PHP 5.2.3 or higher and MySQL 5.0.x or higher, with a web server that plays nice with PHP/MySQL, that you will be fine. Additionally it's recommended to have APC. An ioncube or zend decoder isn't needed, as the source is visible source and not encoded. PHP's sendmail is desirable for sending mail via PHP, but shared hosting usually has that. Oh, and MySQL must have InnoDB table type support.
I've just been searching Google on this and got lead here to this thread. Is there an official xF test script you can upload to check server requirements, check for any possible configuration problems.