Pre-orders and previews underway ahead of launch.
iTunes Music Store - Billboard Charts and iTunes Top 100
The new feature appeared on the front page of the iTunes Music Store last night, and provides yearly charts back to 1946. There are some gaps in the listings where (presumably) the song is not yet offered through the iTunes Music Store.
Apple has also posted a iTunes Top 100 Songs of 2003 compiled from their "most downloaded" records.