I'd say go for iOS 5: according to this article from last year, adoption of iOS 4 was at 90+% about 6 months after release. If iOS 5 follows at the same rate, and considering it was already at 40% only a few months after release, I'd say that the numbers are somewhat similar to iOS 4. The only thing that will drive the numbers slightly down will be the real old devices that won't get iOS 5, but I'd estimate that 75%+ already have iOS 5 and it will keep growing, especially as users replace their old models with new ones.
Edit: found this other about Bump users for December where 5.0.1+5.0 would total slightly over 60%.
Also, if you're new to iPhone Dev, iOS 5 makes it much easier with ARC and storyboards among many other features. Personally, I'm targeting iOS 5 and not looking back. Also, you may want to check an excellent iOS 5 free course in iTunesU from Stanford University: CS193P: iPad and iPhone App Development (Fall 2011)
Final edit: found this article that summarizes it clearly for iOS 4 -
- 1 month after release - 50% adoption rate
- 3 months after release - 70% adoption rate
- 6 months after release - 90% adoption rate