I thought that was a deliberate design decision (also notice that the sky becomes darker as you scroll up). Sure, some kind of min-height would fix the issue, but it's not like there aren't other sites that assume a minimum 500px webpage height.

Same here. (Chrome 5.0.3 on OS X). It's dependent on the site of the window though. I keep my relatively small, so that automatically came to mind.

yes, i see the same, depends on my window size (am on a 1200 x 800 laptop)

Same on my 1024x600 netbook.

Same on FF 3.6.3 on Windows.

A simple solution would be to jack up the z-index on the form so it's always in front of the hills.

Not that I have files on the iPhone to upload, but the background covers most of everything on mobile safari too.

Not here (OS X, tested on Chrome & Safari). What browser/OS are you using?

Sorry yes forgot to include browser details, have amended question