Portrait mode for the desktop seems rather complicated and I don't see it happening. Automatic relocation of your desktop widgets, shortcuts etc? Rotation of your background pictures? Too much hassle for too little benefit? You can always have your frequently used shortcuts in a position that is equally reachable in both modes.

Portrait Dashboard... might make sense if there are more portrait apps that you use frequently. Perhaps it's not that tricky to implement.

... and since both desktop and dashboard are all open source, you will be able to start hacking on them as soon as we release Maemo 5 final. Looking forward to see your proofs of concept!