We noticed this in our application while testing the beta of Q1 2009. Now that we plugged in the release of Q1 2009, the issue remained.
First thing I did before diving into the depths of our code, I checked your web mail demo, which (fortunately for us), displays the same issue.
See the attached image for a 'view' of the issue (screen shots made from the notes part of the web mail demo, but is visible within the entire application):
Basically, when one sets a control to occupy a width of 100% of a RadPane that borders the right side of the browser window, the control is drawn 1px too wide, causing borders to fall outside the visible area, without scroll bars appearing. Interestingly enough, when I selected the shown entry in DebugBar, the issue vanished and the border became visible. When I wanted to take the screen shot for the DebugBar entry, I had to re-size the DebugBar side-bar and, like magic, the border went missing again. The issue was observed in IE7 and FireFox 3.
This is a pressing issue for us, since I have to demo our application framework next week to a very large customer.