Here's what I've found. Others may want to see if their experience is the same if not already tried.
I installed Lion and the new Aperture upgrade two days ago. Nothing has worked properly since.
Yesterday, I could not open the latest upgrade of Aperture at all on my iMac (8gb Ram installed). Like all other apps, it just hung, and Force Quit caused any other app on the machine to freeze up too.
Today, I booted the machine in Safe Mode. Takes about 5 mins (reminiscent of having a Microsoft machine), but Aperture then opened under this method. I can see all projects and albums in the Browser view. In Split View, the thumbnails appear at the foot of the screen as normal, but the viewing pane is blank. In Viewer mode, nothing shows at all.
All metadata is nevertheless visible in Inspector. In Split View, running the mouse over the blank viewing pane in crop or straighten mode affects the thumbnail in the normal way - it crops or straightens.
So, the problem is clearly that the version from which I would normally edit is simply missing from actual view on screen. This suggests the error is partly in the Aperture upgrade, or at least in how it displays under Lion.
I have been through all three of the Library rebuild processes. Neither of these made any difference to the problem. I can view and manipulate separate versions of the same photos in iPhoto, but iPhoto will not read the Aperture library.
Booted in normal mode, Aperture still will not open at all. It says it is accessing the Library, but the Force Quit screen (even after leaving it for 30 minutes) says Aperture is "not responding".
Nothing on my iMac since installing Lion will give me access to any external USB drive or memory card.
Anyone any suggestions, or workarounds that might move this forward, please?