using meta viewport width to scale your website
I realize this thread is a bit old, but perhaps someone else will stumble upon it looking for the answer like I was (which I've subsequently figured out).
Viewport width *does not* indicate the number of pixels to which you want your site scaled (as I originally assumed). Width indicates how many of your site's pixels will be displayed in the device's viewing area.
So f you want to "scale up" a 320px design to fill the iPad:
<meta name="viewport" content="width=320px; user-scalable=0;" />
With this setting, 320px of your site will scale to fill the viewing area on the iPad. The drawback to this is that because the site is scaled it may look blurry (images at least...text should be fine). One way around this is to use higher res images than you need for the iPhone with their size controlled by CSS. For instance, instead of using a 40x40 logo, make it 80x80, but define height:40px/width:40px in your CSS). When your site is scaled from 320 to 1024 (or 768) the image should scale more gracefully. But there's a balance to strike between scaling down an image for iPhone and scaling it up for iPad. It can look bad in either direction so you may need to tinker to find the optimal size.
Best of luck!