I dont think that "Go out and play" is something most parents say to kids anymore. Sadly, I notice that most kids days are full of scheduled, "supervised" activities. Kids cant just yell, "Mom I'm going to go play at Timmy's house!" as they walk out the door anymore. Now they have to schedule "play dates" and be driven over to Timmy's for an hour or so. Sometimes I wonder if my kids will ever be able to have some of the freedoms that I did as a child.

True - I just ran out of the single-wide and had fun. Of course, in a single-wide, you don't have many play options anyway, but still.

Now - there may be one other difference that isn't being taken into account here, and I don't have any statistics to back this. But, two things jump out at me:
1 - We had more 'make-believe' toys back then. There really weren't any video games or computers when I was young. It was army men, matchbox and hot wheels, star wars figures, and the like. Also, it was a hard time economically (at least for my family, think late 70's here), so even if there had been these sorts of toys, we wouldn't have been able to afford them anyway.
2 - Difference between parents and work schedules. I remember hearing about the phenomenon of 'latch key kids' when I was young - and for some of the time, I was one. But for a significant portion of my life, my brother and I had my mother at home (probably up through 4th grade or so for me), so we'd come home all wired up and she'd send us outside so we didn't destroy the house.

couple both of these and you've got some additional reasons why we had more playtime outside when were younger. Kids nowadays can come home and get on their PC - they're relatively quiet and their parents may not be home anyway due to work schedules - therefore, no push to be outside...