At the very bottom of The Verge's review of the recently released Galaxy Nexus you'll find a table of ratings for the device. The very last item rated is "ECOSYSTEM"
The rating is 8 out of 10.
If you shop for and install apps only from, say, Google’s own Android Market and Amazon’s Appstore, how likely are you to encounter [Android malware]?
Perhaps Google Trends would provide a view on how prevalent this issue is:
Well, that's interesting. What about searching just for "ios malware"?
People are discussing malware on Android. They are not discussing it on iOS. This obviously doesn't tell the whole story, but another interesting statistic may be the number of anti-virus/malware apps on each marketplace.
Searching for 'anti virus' on the Android Marketplace...
Not exactly off to a good start. When searching for 'anti virus' on the Android Marketplace, the results were counted as literally 'At least 1000' results. When searching for the same phrase in the iOS App store, there were a total of 35 results, none of which claimed to be any soft of anti-virus or anti-malware app, but rather games or information about web based threats.
I suppose it's a good thing that Google lets anything into the Marketplace. If they didn't developers wouldn't be able to sell their anti-virus apps.