Maybe this will change with the arrival of Gingerbread, which will include NFC support and carrier billing like in Japan, where KDDI has integrated app payment into its carrier billing. Android Market altered its terms for developers in July to include "authorized carriers" as a billing alternative, and considering the US carriers mobile payments venture, this might happen very soon. For now Ideal Binary is holding its horses to enter the Android Market and looking around for other alternatives.
Windows Phone 7 has its Marketplace, which is taking a similar curated approach to its apps catalog as Apple. Therefore the developers from Ideal Binary would rather consider rewriting their software in C#, than dealing with the fragmentation issues and hotch-potch billing policies of Android Market. Still there might be months before WP7 handsets gain enough critical mass to attract the most popular developer houses.
This usually happens when a few million handsets are already in the hands of consumers, unless Microsoft pays developers cash for the popular apps, as was the case before initial launch. Foursquare, for example, didn't plan on porting its app over to WP7 before it hits ten million devices sold, but Redmond just came in and paid for it. This way Microsoft had most of the important stuff covered out of the gate.
Currently there are about 3000 applications and 15 000 developers for Windows Phone 7, but it probably won't be until the middle of 2011 that WP7 devices will pass the ten million mark that is going to attract serious developer attention, so the number of apps can grow exponentially.
source: TheGuardian via WMPoweruser