Getting newest S3 keys first

Question

I am writing an app that stores (potentially millions of) objects in an S3 bucket. My app will take the most recent object (roughly), process it, and write it back to the same bucket. I need a way of accessing keys and naming new objects so that the app can easily get to the newest objects.

I know I can do this properly by putting metadata in SimpleDB, but I don't need hard consistency. It's ok if the app grabs an object that isn't quite the newest. I just need the app to tend to grab new-ish keys instead of old ones. So I'm trying to keep it simple by using S3 alone.

Is there a way to access and sort on S3 meta-data? Or might there be a scheme for naming the objects that would get what I need (since I know S3 lists keys in lexicographic order and boto can handle paging).