When previewing a site from your desktop computer you can select an iPhone or Galaxy Nexus (Android) emulator. BlackBerry and others are on the way.
Tip: Don’t just use this while designing your site! Once you have a great looking site, take a snapshot of it inside the previewer. Then, scroll to the bottom of the site (still inside the previewer), and click the “Desktop” link at the bottom. This opens the desktop URL – still inside the previewer. Now, take a snapshot again. With these two screenshots, you can create a quick “Before and After” slide to present internally, or to your clients!