So, something had somehow become corrupt in feedHopper's database of feeds, causing it to reliably die on launch. Restarting the iPad (v.1 64GB 3G, iOS 5.0.1) had no effect, so I took the Microsoft solution of removing the app within iTunes, then adding it back again.
Unsurprisingly, that's also wiped the errant database, leaving me with just the default feeds.
Is there any way, however arcane, to extract the db from a previous device backup? Or is the only solution just to try remembering all the feeds and adding them back?