Is there any information on which iOS versions are mostly used at this stage?
I am developing a new application, and am wondering with which version I should make the App compatible with (e.g. iOS 3.0, 3.2 or 4.0, the biggest problem being that I don't have an iOS device that still runs lower than 4.2 which makes testing difficult).
Is there a breakdown somewhere which % of devices uses which version?
Purely for what it's worth - just one opinion for you.
As of 2011, many large successful high-selling developers would be going with 4.0.
Updated for Summer 2012:
As of 2012, pretty much all large professional development companies would be going with 5.0.
A comment for February 2014:
It seems to be more true than ever that it is only worth bothering with the most recent version.. Thus currently it is only worth bothering with the latest iOS, namely iOS7. But moreover that is more true today than ever.
Moreover: it is becoming more and more expensive to support the older versions. If you have a iOS7 app, and your client is considering how much it would cost to make it somehow work with iOS6, the answer is "a lot". (Just forget even earlier iOS.) But moreover, this 'go back one' difficulty has now increased tremendously.
Original post:
Traditionally in the Apple universe, there has explicitly not been a concern with legacy.
In the Apple universe, there is more of an expectation that: users are 'sophisticated users' and that users up to date.
(There are any number of historical examples of this: OS-X basically blew away all os9 development, with only a polite veneer of legacy support.)
In contrast, in the Windows universe, and now in the Google universe (Google is the new Windows), there is always a general desire to support legacy. Apple tends to support a clean break with the past, MSFT/Google tend towards accumulation/legacy.
By the way, this is neither "good nor bad." But it's just a fact that the Apple universe "expects users to keep up" (for better or worse) and the Google/Microsoft universe "believes in old-version support" (for better or worse).
Another way to look at it: to be blunt and a bit rude, the few iPhone users who don't upgrade their iPhones (could it be any simpler?) tend not to be sophisticated enough to purchase stuff at the app store.
Penultimately, don't forget that the latest versions are really pretty incredibly more advanced than the previous major release. As a new developer, you will really struggle to program for older versions (ie, avoiding using all the new features in the latest major version).
One final very minor point: the "hard" approach to upgrading is a significant barrier to (A) idiots who try to steal your software and (B) hacking and viruses. (Observe any recent virus fiasco in the 'android' world - something we fight to avoid in the Apple world.) Each new version is that much more proof against jailbreaking, warez, and viruses.
Important ................... Don't forget it will take you a very long time to finish and get your app in store. You may well be looking at the NEXT!! major version.
In a word - if you are new to Apple/iOS business, one of the pleasures is that (very generally speaking) you do not have to worry about legacy nonsense. There's no NT, Vista, W97, etc here :) Forget legacy, and move ahead! Quite simply, in the Apple universe paying customers do not hold on to legacy. You can assume Apple users are paying customers, who move ahead.
Bump CEO David Lieb said in January 2011 that 90% of iOS users had upgraded to >= 4.0. According to their findings, more than 50% used the latest iOS version (4.2.1 at the time).
In March 2011, Marco Arment published great statistics from his app Instapaper 3.0, with among other things, breakdowns of iOS versions, iOS devices and device versions. Showing 98% of all iOS devices (using Instapaper) running at least iOS version 4.0. The then latest iOS version — 4.3, released on March 9th — was used by 65% of all devices when Marco published his blog post on March 24th.
This is a good collection of iOS version statistics, created by Cocoanetics, from August 2011. Their conclusion is that there's no need to support anything below iOS 4.0 anymore.
Update September 2012
One day after the release of iOS 6, David Smith's excellent iOS Version Stats page, reports iOS 6 adoptation of 28%. Smith collects this data from his Audiobooks apps (both iPhone and iPad and both free and paid) with about 100k weekly downloads.
Most of the links in the answers above are broken, so here is another:
http://david-smith.org/blog/2012/03/10/ios-5-dot-1-upgrade-stats/
Also, as noted in this answer, you can get the stats here:
http://marketshare.hitslink.com/browser-market-share.aspx?qprid=2&qpcustomd=1&qptimeframe=M
Supposedly, iOS 5 uses Safari 5.1 while iOS 4 uses Safari 5.0.
Based on there numbers I'm not sure I agree with Joe Blow's point of view. Looks like 20% of users are still on iOS 4 and their share is dropping very slowly, so supporting legacy versions might still be necessary. Depends on your estimated time to market and your target audience.