I wasn’t particularly thrilled that I was forced to drop support for iOS 3, but looking at the number of active users of my app[2] that were running the current version, I saw that none of them were on iOS 3. This was probably the right time for Apple to drop support for < iOS 4.3.
[1] I don’t have stats to back this up, but seeing that iPhone 3GS is the lowest hardware to support iOS 4.3, and they’ve sold way more devices since then than original iPhone and iPhone 3G, I think it’s fair to assume that the vast majority of iOS devices in use can run iOS 4.3 or later.