I got caught up in the iTunes contest and ended up buying an album song-by-song during the 100,000,000 mark. Using these two sites as indicators of when it was going to happen.

I think I got close since my purchases were spread out during the 5 minute update interval. Mat pointed out that it was fortuitous that the switch happened on the eve of MacWorld where they could announce the 100,000,000 mark—as well as the 50,000 song sales sales in the span of about 10 minutes.