A good part of the iPhone's success is similar to the iPod's success ... a nice touchscreen phone that's actually easy to use and looks nice. However, it is expensive.
For those only searching for a phone with a lot of features and a nice interface, Android does seem to be a cheaper alternative that fulfills most of their expectations. Hell, even the Nokia Xpressmusic running Symbian fulfills some of these users' expectations! It is no real surprise that Android handsets are now robbing the iPhone from its target market.
The Blackberry marketshare has been mostly static in the US, though here in Mexico I have seen the 'berries gain ground among first-time smartphone users. I think that BB Messenger and heavy marketing on behalf of both RIM and carriers (as BIS contracts mean more $$$ to them) are the main drive behind BB adoption.
The greatest loser in the smartphone wars is actually WinMo; HTC users are switching to the Android-powered HTC's, while other smartphone users are simply jumping to Android because they fulfill what they wanted and have no need for RDP/Exchange stuff.
Finally, Symbian might still march on, if only because some of the Nokia handsets carry it and so some users not specifically searching for a smartphone will have a Symbian handset without knowing it.