I've waited to say this because I wasn't sure, and then didn't want to kill what appeared to be a golden goose. But all the floor signs now say:

..."Or free with a valid MBTA [T] CharlieCard" ...followed by a plastic CharlieCard laminated to the sign.

It's official: Get a plastic CharlieCard, and ride free between Logan Airport and Back Bay. For a long time - since almost the beginning? - drivers only asked, "CharlieCard?" and waved along persons with one and those with current T passes. To be sure, they'll still accept your $5 by credit or debit card, but they apparently threw in the towel re T bona fides.

CharlieCards can be picked up - free - at the Red/Orange Line Downtown Crossing Station's "T Store" but no longer at Back Bay Station, per the MBTA website. They once were available from other stations' attendants, but I don't know if this is still the case.

...The best news for frequent travelers since [your item here]. But it can't last longer than the Government Center Station project ... All that money we save isn't going to someone.