What happened to the ‘preview’ window in Blender 2.5 (the one that you could overlay and resize) and the other render preview options (like ‘render portion’)?
I can’t seem to find a simile… 
The render preview doesn’t exist.
Border select to render is Alt+B+drag mouse. The render settings are under the render tab in the properties window.
They are not included in the 2.53 beta. Hopefully they will come back in future to the official release.
Given just how awesome everything else is, it’s a small price! 
Thanks for the info.
*Shift+B+drag mouse worked for me, just in case anyone ran into the same problem as I! 