Interestingly, on first launch of the new beta you're presented with the dialog on the right. Apparently DL2's scrolling and display performance relies on fixes delivered in Mac OS X 10.5.3 -- that is, fixes you can't get yet in an OS build that hasn't shipped. Patience is a virtue, I suppose.
Update: Wil Shipley replies below. The beta test is of the integrated store functionality in DL2; the software itself is not launched yet, so be cautious.