You and mdasen both have a valid theory. Unfortunately it's just that, a theory.
Tangentially is Apple losing their acclaimed grip on the supply chain if they can't make enough iPhones but Samsung can make and sell double that after paying sales people more and still making 7.4B in profit?
Similarly, if Samsung had a press event a week after Apple introduced the iPhone, it would be prudent to wait and see if Samsung had something amazing that they were introducing.
The first theory assumes that some consumers will wait until both phone are available to make a choice. Obviously, some will choose iPhone and some S3.
The second theory says that some people will go to buy an iPhone, and instead get an S3 when no iPhones are in stock. Again, plausible. This comment has nothing to do with which phone is better.
Help me, what part of that doesn't follow?
Absolutely false. I merely point out that many people were looking for an iphone.
"Tangentially is Apple losing their acclaimed grip on the supply chain if they can't make enough iPhones"
No. New iPhones have always been supply constrained because of huge demand. Sprint may have some stock because Sprint's network is shit in many areas.
Both theories are probable enough that they likely happened in tandem - with some people waiting to see Apple's phone before pulling the trigger, others heading for Apple and being sold a Samsung. We could probably figure the quantity of each by comparing sales variations by brand and model.
When someone comes in the store asking for an iPhone 5, the salesperson is not going to push some cheap crap on them. They will push the best, most expensive alternative. As far as I can see the Galaxy S III is pretty much the only phone marketed on eye level with the iPhone by carriers.