Obviously, the iPhone 5 isn't out yet. But the fact that these advertisements persist means that someone thinks they are worth the money. One theory is that the ads exist to drive traffic to the iPhone 4 order page, which is where the link takes you. It could also simply be a cunning prank.
It's also possible that this is being done by Apple out of desperation. If they really are in the middle of a production bottleneck bad enough to delay the iPhone 5 until 2012, Apple has a serious problem on their hands. iPhone 4 sales are going to fall off as soon as we start hearing solid info on its successor. If Apple has to delay the iPhone 5, they'll also need to keep people interested in the iPhone 4 until then.
Buyers interested in the iPhone 5 search, see the Amazon link, and end up looking at the tantalizing iPhone 4. It's a clever plan- but also unlikely. Apple invested millions into securing components for the iPhone 5 and iPad 2 well ahead of time. They are the least likely company in the industry to run into a serious production issue. But even Apple isn't immune to a devastating earthquake. It could be their upfront cash offers haven't been enough to meet component demand.
There are several rumored iPhone 5 features that could be the source of a bottleneck. We've long expected an NFC reader, and the possible 4" touchscreen could be more difficult to secure parts for. Apple may also be hard-up on the new dual-core A5 processor. The iPad 2's wild success can't help but have an impact on supply.
A 2009 Apple patent has sparked speculation of a secondary display on the iPhone 5. This would add touch-sensitive controls to the edge of the display and around on the back. Considering the constrained supply of touchscreens right now, Apple may be feeling the sting of their ambitious hardware plans.
Of course, we still haven't even gotten to June. Apple has probably upped security since the iPhone 4 leak. All this speculation may just be the inevitable result of a vacuum of real leaks. If I were a betting man, that's the horse I would back.