The new OS (3.0+) adds more features which speed up the development (like working with the lists etc). On the other hand there could be (potentially) a lot of users somewhere still running OS 2.2.
Is there any place to look for statistics? Could someone offer an advice on the target OS for a new iPhone app scheduled for Christmas release?
Given this data, if you're only supporting iPhones there's no reason not to go 3.0 only. If iPod touches are an important part of your audience, it may by a harder decision.
However, I'd wager that the people willing to pay for your app are also willing to pay for the 3.0 upgrade for their iPod touch, so they can get the latest and greatest app.
Finally, in my opinion, if there's a significant draw to 3.0 for development reasons, I'd say go for it. I'm writing a 3.0 iPhone app at the moment, due for release in the next few weeks, and I didn't even consider 2.x - Core Data is too much of a draw for me, even though I can achieve persistence with SQLite.
It depends. If you want to take the advantage of the new features you must use the SDK 3, if you don't need them you can just stick to the old one.
The problem why users stick to old soft has two sources in my opinion:
Based on Admob data, adoption rates of the earlier SDK releases have been pretty good in past. It takes a couple of months to reach over 60% adoption. See Admob stats
On the otherhand, I just saw another Admob report, which indicated that iPod Touch users are more likely to buy apps and they are not that likely to upgrade.
However, I'd bet on 3.0 for Christmas market and next year, if you need something from 3.0 SDK.