USAToday reports that Amazon has managed to take the #2 spot amongst digital music retailers behind iTunes with their MP3 Store. No sales numbers have been released, so it's difficult to gauge exactly how close a competitor Amazon is becoming.
This information comes from the four major labels who played their part in making it so. Amazon has been provided access to a larger number of unprotected music from the major labels to stock their MP3 Store.
Apple now has 2 million songs from EMI and independent labels available without DRM, out of its 6 million-song catalog. Amazon offers 4.5 million DRM-free songs.
For its part, Amazon has done well providing seamless integration with Apple's iTunes software on the Mac.