Apple controls the update 100%. Apple has successfully stolen the customer relationship from the carriers. BlackBerry has done this to a small extent but their primary customers are still the carriers.
Google, with Android, has ceded all control to the carriers and the handset makers. For example: Customer walks into BestBuy and asks "I want a Droid". Instantly, they are shown only the Verizon Android phones with the "Droid" branding.
iSmashiPhone took a look at mobile operating systems and their update schedule. After plotting it on a tidy chart, noticed a pattern of consistent and regular updates to iOS especially when compared to the much more, ahem, sporadic updates of all the other mobile operating systems. The site notes, that after a slow start, Android is gaining ground.
The exception now may be Android, which is picking up the pace. However, a look at the other major mobile OSes will show how infrequently they tend to be updated.
It deftly concludes that frequent OS updates are key in the mobile software race noting that Apple has lead in this regard from the beginning.
Android is catching up, and their recent release schedule shows that they “get it” as far as updates go, but RIM and all the other guys are still lagging way behind.
Okay, most of the one I know have now upgraded to Vista or Windows 7, because the old machines finally gave out and they only get updates on them, because I set them up and set them to download and install automatically! 
