Avoiding leaks: Strategic Apple
Apple avoids leaks by working on all facets of their product lines.
They develop the iBooks, iPods, iMacs, etc and any new products simultaneously. Steve jobs just picks which ones are ready and have been developed enough to be released. Even more so, he picks a conservative lineup of products, while certain products may outshine others, there is a recipe to get full impact and attention for a product. (Apple is not alone at this, in fact any good innovative company have their own recipes of success with product releases, but unfortunately even good companies dont.)
Rumour sites may obtain accurate info, but find that the product maybe released a few months or even a year after. But more often than not, rumours are just that... rumours.
Apple is great at being very receptive and strategic when preparing for their product releases. Each release is a stepping stone that always have an ulterior motive:
MacBook pro (powerbook equivalent) paving the way for a MacBook (ibook equivalent.
iMac Intel Core Duo may lead to the renaming of powermacs into... iMac Pro (a quad version of Intel most eminent).
Leaks or not, it is paramount what we can expect from Apple... QUALITY. Very few companies maintain a level of innovation that ensures quality in their product, though most claim to do so (flaming windows right now, ahem).