The exact same thing happened to me yesterday! Unheard of. I felt like it was a prank too, but seeing your post was reassuring
"I am pleased to inform you, that due to an unexpected number of students declining their OGS offer, you have been moved from the unsuccessful list, and are now being offered an OGS award for your 2012-13 graduate level studies. Congratulations!"
I wonder how many other unsuccessful applicants were moved up as well