iPad Peek is a nifty tool that lets you see how any website will be rendered on the iPad. Click on the top border to switch from landscape to portrait mode. The virtual keyboard and the buttons on the iPad browser are just for show, but the reload button works.
Previewing a website in iPad Peek doesn't offer the "real" iPad experience, however; for example, Flash works (and it shouldn't). To get closer to the real deal, you should disable the Flash plugin in your browser and change the user agent string to that of the Apple iPad.
To do this last bit in Firefox, type "about:config" in the address bar, click the right mouse button, select New - String, and name it "general.useragent.override". Then enter the value "Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.10".