[appigo-todo] todo deleted itself, can't restore

My iPhone is OS 3.0.1. I don't wish to upgrade at this point.

Todo has disappeared from my phone, and won't sync from iTunes.

I've restored the old todo version from a backup (music/itunes/mobile
applications/todo 3.0.1.ipa) but I'm still getting the error that it
requires a newer version of iPhone os to work.

Help! Todo used to work, but now I can't install it at all, and
suspect all my data has been removed too. At this point I'll take the
data loss but don't want to purchase a different app that will work,
when this had been working so well.