We already get big gaps from announcements to availablilty, the Mark II big whites were announced in August 2010, and have only been available in the UK a short time, so nearly 2 years.
Personally I would prefer they delay announcements until they have stock ready to ship. If I see an interesting item announced that I want buy, I want to be able to buy it straight away, and not have what feels like a never ending wait for it to come to the shops.
I have decided to change my 50mm lens for a 24-70 MKII (because of rumors I knew it was on the cards), but by the time it comes to the UK (I expect Jan 2013) that decision will have been made 2 years ago.
I think rumors is excellent for planning the longer term changes to your camera system, and possibly avoid some shorter purchases that have to be replaced.