While erniadeldesktop is pondering the finer points of Euclidean geometry I'm still unable to start the app....

In this case, I hadn't changed username but had saved the deb via GUI/filemanager to N900 / Documents.

I copied the deb to /home/user in Terminal (see screenie), tried

sudo gainroot
dpkg -i /home/user todoy0.0.1-1-all.deb

and the deb was installed *somewhere*...but it still just closes a few seconds after opening.