I know - we made the mistake of the setting the target platform to iOS 3.1 but only ended up testing it under iOs 4.0 devices - simply because no-one seemed to use iOS 3.1.
Wrong !
Although 93.6% of our users use iOs4, the other 6% were pissed off when they found out that the app crashed on iOS 3.1
The icing on the cake ? After the app crashed, the users deleted it - and were prompted to rate it. I'm sure that you can imagine the rest.
Thank god that the 'rate on delete' feature was removed from iOS4 - although that is scant consolation.