the last time I was looking so carefully was the 2011 MBA refresh and if memory serves me right, components and features (like the back-lit keyboard) started floating around a couple of weeks before the release...
Lets look at other 'indicators'
(1) Bespoke spec shipping dates slip - No (have been checking and certainly in the UK can get most specs with 1 day lags)
(2) Inventory shortages at 3rd party sellers reported around the community - No, the likes of Amazon still hold plenty of stock
(3) Rumours of release date start getting more and more specific - No (certainly not that I have seen in my channel checks)
(4) Staffing levels / holiday plans / etc at apple stores come out - No
(5) Store display fit-out dates (overnight) info starts coming out - No
(6) Inventory numbers start filtering out - No
So why does everyone think the refresh is so imminent? Because intel released a new chip? Last time there was an iMac refresh, it happened weeks (if not months) after the intel chip was released.