The current selection could be in Dlg.Control(ctl.name).value since beta5, or it is in .index or selectedIndex, don't know what Jon decided on. How to create listitems could be described in the updated release notes. Dlg.Control("lb1").AddItem(label, value) should work, as should something like Dlg.Control("lb1").AddItem( ..CreateNewListItem(label, value) ), but I'd need to seek through the release notes pdf here as well to tell for sure.
PrePostEdit:
New API does not seem to be described in the release notes pdf yet, so maybe wait for the devs to tell you. o)