It isn't new - I nearly got bounced coming over for Boskone in 1993. Fortunately I was with others (thus adding plausibility to my story about attending a con) and I hadn't stayed long on any of my previous visits.

Plus I explained the profusion of BOS and MSP entry stamps as being due to having my frequent flyer account with Northwest, and this rang true with him.

Even if I get my Green Card, it's still true - I can be bounced on arrival in the US, though I may have a right of appeal then.