iPad Peek has a few limitations. There’s no touch scrolling, ads produce pop-ups, and embedded Flash videos and objects will still render inside the emulator even though the real iPad doesn’t do Flash. So, it’s basically a Webkit wrapper set to a fixed width and height. But, it does give you a pretty close approximation of how your site will look on the new shiny.
Also, Mashable has instructions for changing your Firefox user input string to that of the iPad:
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″.