Amazon MP3 and Walmart weren't immune to pressure for variable song pricing and today started quietly charging higher prices for popular songs. In particular, about 10 of the top 100 songs on Amazon now meet the same $1.29 prices as similar tracks on iTunes, including recent Beyonce, Britney Spears and The Fray songs. Walmart's store, in turn, is now charging $1.24 per song for some titles and has a section dedicated to tracks at the higher price, most of which have been long-term hits rather than just recent releases.

Lower prices are also in effect for older or less popular songs, with some tracks selling for $0.79 at Amazon while Walmart carries others for as little as $0.64.

The quiet price hike confirms that Apple's unofficial switchover date for variable pricing on its store is part of a larger move to raise prices across the industry. Concerns had been raised that the music industry might target Apple specifically after an initial comparison of prices this morning had suggested only iTunes was carrying the higher prices. Major music labels have publicly expressed a desire for such pricing after the emphasis on single downloads over albums, as well as plummeting CD sales, hurt the traditional music industry as a whole.

Apple chief Steve Jobs and other critics of major music labels have actively resisted variable pricing in the past as too confusing to shoppers and more likely to discourage them from buying expensive new releases. The company's agreement to allow flexible prices is known to have come in exchange for allowing all songs to be sold without copy protection, a sticking point which Universal, Warner and others have used as leverage by giving unprotected tracks early to Amazon, Walmart and others while withholding them from Apple until it gave in to their terms.