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.
 Full disclosure: I haven't released my first app yet so I'm wondering if I should only develop with the new IOS 5 tools or if I'm going to have to write my app with Xcode 4.1 anyway.
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+.
There's only one reason that I can think of: Verizon/CDMA iPhones only run iOS 4.2.x, and won't be on parity with GSM iPhones until iOS 5 comes out.
For whatever reasons, some users still stick with older firmware versions. 100% of our userbase is iOS 4+, but about 10% of it is below 4.3.