Apple used to introduce many new products at the Macworld conventions, in San Francisco, New York, and Boston. Since Apple has completely backed out of Macworld, they no longer introduce new products at Macworld.
Apple has several different options for introducing new products (hardware):
1. WWDC (Worldwide Developer's Conference) - usually in June. Apple will introduce products at WWDC very often, usually they introduce something new there, the last year it was the iPhone 4
2. Special Event (like the iTunes/iPod event). Apple will host a special media event somewhere like the Yerba Buena Center for the Arts and introduce products there. This usually happens around October
3. Special Apple Event (on Apple campus - invite only). Apple will hold a special media event at their campus in Cupertino and introduce new products there. Usually a very small, invite audience in a small room.
4. Silent Product update: Apple will simply introduce something new and post it to the Apple store and their website, with no formal event, like the recent new Mac Mini or Mac Pro. There was no event to unveil these machines. These were simply silent updates. This may likely be how the new MacBook Pro is announced.
My bet, is if they re-design the case design and make major changes, Apple will probably hold a special event around April or May to introduce the new MacBook Pros. That is my best best. If not, they may just hold out until WWDC in June and introduce then. That is very likely as well.