Google Trends data is fascinating. It's very partial, and in particular doesn't capture app store searches, but it can still give a very good indication of where things are heading.
This chart shows search volumes for "BBM for iPhone" (red) and "BBM for Android" (blue). The hypothesis is that when a BBM user dumps their Blackberry and gets a new phone on a new platform, they Google to see how to get BBM on that new platform (they can't).
Hence, this chart illustrates the way that RIM lost high-end customers first, in developed markets, who went to the iPhone. Then, it started losing lower-end customers (in emerging markets), who went to Android. The high end departure looks like it's mostly over (though this obviously doesn't tell us how many loyalists have stayed) - the departure to Android is still going on.
Google Trends also gives regional interest data, which supports this interpretation: "BBM for Android" (blue) clusters in Indonesia (not a big surprise), "BBM for iPhone" (red) in Canada and the UK, and more recently South Africa.