Now, there is a practical reason why these distrobutions are released in .IMG form -- .IMG files are apparently intended to be immediately flashed onto USB thumb drives, since most netbooks don't have CD-ROM's at all (the usual destination of an .ISO file). However, I haven't found a reliable way to use these images as a boot source in Virtualbox yet.
I've found where some people have said just rename it from ".img" to ".iso". That didn't work. Some people said try mounting it in Virtualbox's virtual floppy drive, which normally takes .IMG's anyway -- that didn't work.
I know I can just burn it to a USB flash drive that I have handy (if I had an empty one handy :P ), but that defeats the whole purpose of being able to test all this stuff in Virtualbox without having to rely on external physical media!
Oddly, the best solution I found was present at the Debian.org website:
VBoxManage convertfromraw -format VDI [filename].img [filename].vdi
Just use that command (replacing "[filename]", of course). This will convert the .IMG file into a Virtualbox .VDI disk image -- now, simply add that disk image as another hard drive, not a mounted ISO. Here's a pic of the Virtualbox config screen I'm talking about:
Now, just start up your Virtualbox VM and press "F12" to choose your boot drive, and select the drive that you've configured as the converted .IMG.
Whew! Lot of steps for something which is supposed to make stuff quicker! :P But it works, and I'm sure there's an easier way -- they're always is.
That being said, I was pleasantly surprised with both netbook OS's. Ars Technica has a great writeup on Moblin here -- the UI is just amazing. You need a modern computer for it to work on (a CPU with SSE3, at least, I think), and my old Compaq EVO410c from 2001 just didn't cut it, so I'll be sticking with the Ubuntu Netbook Remix.