I'm all in favor of anything that puts upgrade pressure on users of IE 6 and 7, but this rolling "current and previous major releases" idea is silly. They should simply establish minimum browser requirements like everybody else and update them when it makes sense to, not arbitrarily leaving it to whenever the browser makers release new versions.
"Current and previous major releases" is a peaceful approach. There is no discretion involved - at no point was an active decision made to cut off your IE 7 users.
Plus, your FF 4 users will know exactly when FF support will be dropped - the day FF 6 is released. Simple.
