I don't know if you can configure dual monitors through Xrandr while using a proprietary ATI video driver. I don't use a dual monitor setup. Because they were able to change the MapToOutput code to handle the Nvidia proprietary driver it must be possible. Since I think they could work with the ATI driver without the code changes that may mean the ATI proprietary driver is compliant enough with Xrandr to use it. For all I know that's what Catalyst is using.
That's too bad on the ASUS R1E, being stuck with Hardy. That and a Gateway were the two other tablet PCs I was seriously considering. Lucked out choosing the HP instead. Besides the deal was too good to pass up.