Given this has been coming for a few months and many of the Obama supporters are practically worshipers, It doesn't surprise me one bit they could get 70,000 democrats to show up. Looking at the numbers, that really isn't a whole lot. I heard they were expecting to issue 15,000 media credentials. I don't know what the final number was.
Remember Dan's Bake Sale that Rush put on back in the 90's? That event attracted a whole boat load of people for no real reason at all (wiki says 80,000).