Standard exceptions to the rule in recent memory include:
- products that depend on 3rd-party developer support. If the iPad 3 has a new form factor and new hardware such that developers will want to rewrite their apps to take best advantage, Apple might show it at the developer conference and tell everybody "you've got 3 months to get your apps ready"
- products that get accidentally leaked or need to be leaked early due to regulatory requirements such as filing for FCC approval.
What Apple doesn't do is float trial balloons just to have something to show. Lot of companies will mock up some design that is approximately similar to the expected final product and show/announce that, then finish figuring out how to build it. Apple's ideal is to make a big splash by showing something unexpected that you can buy in the very very near future so people run out and buy it, garnering additional publicity based on the big lines and big numbers of customers.
Which is great if you can pull it off, though it seems inevitable that one of these days they'll guess wrong. If and when they do blow it, it'll be at least a billion-dollar mistake to have built up so much inventory in advance of finding out whether people like the thing.
Ready to ship almost never means "in stores". Additionally, Apple trusts no one, even when NDAs are in place, so at the very least the product has to be ready to go to third-party partners by the time it's announced. Often, the partners may need some time before it's "ready to ship", which can account for part of the delay.
That said, last minute bugs do happen and cause delays. Apple will ship late before it ships a known defective product (yes, it has shipped defective products, but not any that were known defective), but it doesn't like to do either.
Hm. I guess the iPad that shipped a couple of months after being shown is the special butterfly exception that confirms your nice fluffy rule.