Friend's in Epcot today, sent me an insane picture of a line just to get into the MouseGear store stretching down past the fountain in front of Spaceship Earth. Apparently some of the 30th anniversary merchandise, including some Vinylmations, sold out within minutes of park opening (cast member told them that the only people who had a chance were those who were lined up before 7am to get stuff). Sounded pretty chaotic. Some of the merchandise was on sale earlier (phone cases, some pins, t-shirts), but some just went on sale today.
BTW, the reason the shirts are 6-8 week delivery is because they haven't been printed yet (I think this was indicated in one of the responses on the Disney parks official blog somewhere, I know I read it earlier). I don't think they will "sell out" online because they are making exactly what they need to fulfill the orders. The only ones they printed already were the ones they're selling in the park.