[groovico]

If your running on HTTPS for checkout make sure the CSS reference you are using is not absolute, i.e if you have it as http:blah.com/skin1/skin.css it won't load if your security settings block non secure elements.

Also worthwhile checking it from another net connection incase your ISP has an erraneous Cache of it. I've had ISP's cache sites for 48 hours to deal with their user loads which can cause chaos when developing a site and you don't see the latest update. (an ISP can cache a page even it the headers say not to)