Hassan, Kevin, Jason know I would suggest Photoshop as well, Use Photoshop to not only create a layout, but when the layout is done, use it as a guide to create new components, Both hassan and jason have done this well before, you can always ask them for a detailed how to. @ one point hassan made some flawless Iphone components out of photoshopped images that acted in the way Iphone's GUI did (@ Hassan I still want to finish that pack and submit them here for users)
Point is, it is really your best bet for truly custom GUI