A mobile analytics firm named Flurry assembled this chart, plotting the percentage of newly registered iPhone and Android app projects over time.
The rise and fall of each platform feels like a blow by blow of major product announcements/launches.
The January Apple app development boost, for instance, is attributed to the iPad (despite it being announced in the waning days of the month). While, it would probably be safe to attribute Android's strong December to the Droid and maybe even teases of the Nexus One. Android's July spike gets a bit more tricky, but the European release of the HTC Hero may have something to do with it.