Depends on the situation. If you're the leader, it doesn't make sense to leak the juicy info that could cause people from buying your current offerings.
However, if you're new to the game, or have lagging sales, then yes. Leak the juicy details in order to get folks to not buy the competition and wait for your stuff.
Apple, however, operates differently than just about everyone else. They like to build hype by keeping as much secret and hidden as possible. Then upon the introduction, everyone goes nuts over the new product or features. Over the years, it's gotten harder and harder to keep this stuff secret. The last big secret reveal that I could remember was the iPad 2. Apple made it so thin and capable, that everyone else just threw away their iPad 1 competitors and started over.
EDIT - also, not sure how old any of you guys are, but there's a term called "Osborning". Refers to an old computer company that basically killed themselves because they announced the Osborne 2 so early, that no one bought the Osborne 1. Which caused them to not have enough revenue/profit to build the Osborne 2, which never came out.