There's now quite a bit of conflicting data in the US market about loyalty, satisfaction and intended future smartphone purchases. See, for example:

• Survey: Smartphone Market Still Up for Grabs
• Report: Android Making Big Satisfaction and Purchase Intent Gains -- or Is It?
• Poll suggests third of Android owners really want an iPhone

Nielsen has come out with some new survey based data that argues Android is starting to reach parity with the iPhone in terms of “most desired” next device and among men it slightly exceeds the iPhone:

Here are some of the findings from the firm's quarterly survey of just under 20K mobile users: 

• Current smartphone owners planning to upgrade showed a preference for the iPhone 35% vs. 28% for Android
• The iPhone and Android were “most desired” "among likely smartphone upgraders, with Apple showing a slight lead among those age 55+ , 18 to 24, and 25 to 34"
• By contrast a larger percentage of featurephone users planning to upgrade to smartphones were less sure of which brand to buy (25% vs. 13% among current smartphone owners)

More interesting and important than the perpetual iPhone vs. Android discussion is the data showing smartphone penetration very close to 30% in the US. The data also show the iPhone and RIM now with an equivalent share of devices (27.9%) with Android not far behind (22%). It's not yet clear how well Windows Phone 7 is doing. There's some anecodtal evidence that it's not selling well despite generally positive reviews. 

Again, these data are self-reported (and not based on unit sales) but the sample size was huge and so we can fairly reliably generalize the findings to the broader market.