i've yet to do any full layouts with wordpress other then edit and make images for people. I doubt it would be difficult at all. although the out of the box administration ability and module positions you'd get with a joomla design from me wouldn't be present with wordpress. Its framework is way less advanced than Joomla, geared toward simple
blogs with some extra pages. I'm basically geared toward making images for people or making them a layout that has dynamic capabilities without having to know much html or php. I'd like to do a nice wordpress design though, just to say i did.