Recent survey data from Consumer Intelligence Research Partners shows that 42% of those in the U.S. that bought bought an iPhone in April were fleeing from another smartphone platform, 38% specifically from Android and BlackBerry. It's worth noting that the sample size was only around 521, which might not be big enough to be globally representative, depending on your standards, plus that sample was weeded out from an initial batch of 7,348 might color the results further. Still, I think the general sentiment that iPhone is roping in lots of folks from other platforms holds water.
Though this is only a measure of the gross influx of iPhone users, I doubt there are many outbound considering Apple's traditionally high loyalty rates. The research also shows that for 2% of buyers, the iPhone is their first mobile, and 22% were migrating from feature phones. That's a pretty small proportion, considering smartphone penetration in the U.S. is still under 50% in the U.S. Sure, it's a big leap to make from dumbphone to smartphone, and maybe the iPhone's adoption rate of first-timers is better than that of other platforms, but I would have guessed that iPhone would be pulling in more noobies than smartphone veterans.
Which smartphone platform did you come from before iPhone? Do you know anyone still packing a feature phone? You've been prodding them to get an iPhone, right?