I'm having the same problem on my nvidia 6800NU, 7600GT, and 8800GT.
The admin at powerstrip thinks that xenarc has never actually achieved native resolution on their monitors and I'm starting to think he's right. I called xenarc support two weeks ago and left a message but never heard back from them. Personally, I think Xenarc screwed everyone over. I'd love to be proved wrong. A simple "800x480 works on THIS video card" would do the trick but they can't even do that.
With the onboard video on my Biostar p480-m4 I can output the 800 x 480 but the monitor only sees it at 640x480 @ 61hz and requires a manual refresh every time to show the image rather than skew it off to the left. The dimensions are correct (or close to) but the image looks like crap. Xenarc said that's what it displays when you have the correct resolution but that's a lie and everyone knows it.