Between iOS and Android, I noticed that bestseller apps get ported to Android more often but the traffic is pretty much one way, only a handful of Android apps are ported to iOS, indicating that Apple's apps are indeed superior. It's obvious Apple's ecosystem is more mature than Android but it seems Android closes the gap a little more than iOS increases it with it's yearly updates.
Another indicator is that, here in the US, the desire for iphones was strong enough to drive ~1 million T-Mobile users, including me, to buy it without subsidy, knowing they will have crippled service. No single Android device had similar impact and numbers of users doing that for any Android phones are minimal. Granted, the need for that is minimal since Android is completely carrier agnostic. There is however a trend that started this year, indicating several Android models, most notably the SGS2 and the fall/winter lineup that's really stirring up the battle in the high end Android segment.
The time that takes Android to catch up with Apple's latest models is decreasing as well, due to Android maturing as well as better hardware/features. I would argue that it took Android almost a year to break even with/beat the iphone 4 with the SGS2.
The iphone 4S obviously isn't an iphone 5 but we can still hypothesize that the G Nexus would at least be a worthy opponent to the iphone 5, a mere 2/5 months after it's launch, in a scenario in which the iphone 5 came out in June/September, which as we know is a lot less iphone "domination time" than we saw with past models.
It's up for argument how long it will take or even if Android will ever have an app/game (even temporarily) exclusive that's hot enough for many iOS users to wish for it to be ported. IMO, I think we might see that happen sometime next year.