Ben Lorica of O’Reilly Radar has what I think is a contentious post where he has analysed newsgroups for references to mobile platform names from which he has tried to infer developer interest.
I don’t think his results will be very accurate. Firstly, developers don’t just use newsgroups any more. Most use web sites, wikis and forums. Secondly, the Apple SDK agreement prohibits people from talking about it – strange I know, because you would think people helping each other would help grow the platform. Thirdly, developers who already know about a platform inside out (e.g. Symbian, Windows Mobile) will seek out much less information.
Chatting with various developers, it seems most are surprised by the success of new iPhone app ecosystem. On TechCrunch you can view a periodically updated spreadsheet of how well the top 50 developers are doing. Note however, that the values are for the US app store only. Other countries have their own stores. Hence, the values are actually higher than shown. PinchMedia have a running commentary on how the iPhone app store is evolving (thanks to Sander for the links).