Since the other thread was locked, I'll re-iterate what I said here.
I'm surprised it happened this quickly, but most of the credit goes to Verizon for aggressive promotion and 2-for-1 deals. RIM used a similar approach for big marketshare gains last year (hi, superchunk!).
I agree with mrstickball that growing the software marketplace is the biggest challenge for Android right now. Their second biggest challenge is building a worldwide market presence, where Apple still seems far ahead.
No matter how sweet their deal with AT&T, Apple should seriously consider opening up their carriers in the US. Locking themselves to just one carrier has clearly left a huge opening for a strong competitor like Android to exploit.
To give an impression of how much catching up Android still has to do, here's a chart that shows that the iPod Touch alone still sees more usage than Android. Though it wasn't that long ago that Android was sitting below WinMo. Android will probably overtake the Touch in a month or two:
http://marketshare.hitslink.com/operating-system-market-share.aspx?qprid=8
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