XUL will rule
The most significant thing here is this running of FireFox safely.
Have you guys seen what's possible with XUL. We're just starting with it but basically it's a full on rich client which you run in FF - and that's FF running on *any* platform.
OK - so, for a general web application for the masses use LAMP, Zend framework, MVC, Smarty, REST etc. Cool solid cross platform app for easy access to anybody. Keep the AJAX to a minimum.
Where you want more detailed control say for administrators use XUL. Your application can then be be installed as a FF add-on or I think can just be accessed as a normal web page from Firefox. Certainly the buttons and other widgets are created purely with XML which is viewed in the browser.
So, HP supply Windows or SLED machines - doesn't matter which cos the user is going to be using the company application which will run inside FireFox.
Updates are either via FF add-ons update or updated code at the server.
As for the choice of OS - we have a first Ubuntu machine at a clients sitting in amongst the Windows XP machines. Feedback so far - the Windows users are saying how fast the Ubuntu machine is (same hardware specs as theirs).