It's a universal app, so this covers iPhone and iPad. Over 80% of users are on 5.0 or above.
My take on this is that if there is any iOS 5 feature that would make your life easier and save you time, or make your user's experience better, then go for it and don't worry about backwards compatibility. But if there's nothing specific drawing you to iOS5 go ahead and support old versions, it's not that much extra work to test and support older versions.
Here's one thing to think about: What does you app do? If it's something that is going to run poorly on older devices, the ones that aren't upgradable past iOS 4, then I would say only target iOS 5. More people are updgrading their devices every day and that means they're getting the latest version of the operating system. If you'd like to rely on rumor, think of all the people that will be upgrading after WWDC in a few weeks.
I say go for it, you'll have a large enough install space as it is with iOS 5, I wouldn't worry about your lost users.
I launched my app (Finderous) only on ios 5.x becuase
1. I didn't want to support an OS that is already shrinking in usage.
2. It's just a matter of time people will upgrade to IOS 5.x as it is free.
3. I learnt ios programming only on 5.x
4. I found it time consuming to learn pre 5.x programming once I started with 5.x :)
Not everyone can upgrade to iOS 5.x for free. I had a first-gen iPod which couldn't upgrade to 4.x. The older devices have limitations on which iOS upgrades they can get.
My guess is that the numbers will change significantly in the next few days as a large percentage of users holding on to 4.3.3 are doing so because it was the last easy untethered jailbreak. Now that 5.1.1 is jailbreakable many users will upgrade.
http://imgur.com/Mu7Tp
It's a universal app, so this covers iPhone and iPad. Over 80% of users are on 5.0 or above.
My take on this is that if there is any iOS 5 feature that would make your life easier and save you time, or make your user's experience better, then go for it and don't worry about backwards compatibility. But if there's nothing specific drawing you to iOS5 go ahead and support old versions, it's not that much extra work to test and support older versions.