Carrie Underwood’s aptly-titled Play On has landed at #1 on the new Billboard Top 200 Albums chart this week, after sales of roughly 318,000 copies. The “Cowboy Casanova” singer now has the distinction of having the best opening sales week of 2009 by a female artist. But if those 318,00 folks who picked Play On just can’t get enough of the American Idol Season 4 champ, she’s set to co-host the Country Music Awards tonight with Brad Paisley.

In another turn of chart-related events, industry mag Billboard has now retooled its main album chart rules. Up till now, if an LP released over 18 months ago dipped below the #100 mark on the main tally and no longer had a current hit single on the radio, it was no longer eligible to register on the Top 200 Albums chart.

Hence the reason various Michael Jackson albums—such as his 2003 Number Ones compilation—sold bucketloads over the summer, but were relegated to the Top Pop Catalog chart. To date Number Ones is the best-selling U.S. album of 2009.

Now, if only these new rules had been implemented a couple months back…then we could have seen the Beatles bombard the upper reaches of the Top 200 Albums.

Ah, well. There’s always “Beatles Night” on American Idol to rely on next year. Maybe that’ll do it.