ARC includes zeroing weak references, which need runtime support. These are only available on iOS 5.
The rest of ARC still needs a bit of runtime support, but the functions it needs are basically just wrappers around the standard Objective-C memory management messages. They exist because they enable various optimizations. Apple provides a shim library for these which allow ARC apps to run on iOS 4.3. I'd guess they could run on even earlier OSes, but I believe that's not officially supported.
This is just my opinion, so don't take my word for it.
There is no reason to support anything earlier than iOS 4.3 for an app being released today. 4.3 works on all devices except the original iPhone, the 3G, and the equivalent iPod Touch models. It's also a free upgrade for (nearly?) everybody. The fraction of your audience still using something prior to 4.3 will be very small.
If you're not releasing very soon, it will probably be worthwhile to simply require iOS 5. New OS version uptake tends to be fast, and it seems likely that those who don't update their OS are also less likely to buy apps. iOS 5 is a pretty big deal with a lot of new features, so I think it will probably see pretty swift uptake. iOS 5 also provides a lot of nice features for a developer (like those zeroing weak references, which I believe are pretty important for ARC programming), so there's a good incentive on your end to require 5+.
Good point, I forgot about that. I would argue that, with the small number of CDMA iPhones sold so far, and with iOS 5 just around the corner (maybe as early as next week), they can be ignored for this purpose.