One reader notes that at least one iTunes Music Store song is being offered at a much higher encoding rate than Apple has publicly advertised.
The majority of iTunes Music Store songs come in 128kbit Protected AAC audio file format. This file format allows for playback in iTunes as well as the iPod. The 128k bit-rate in AAC is said to provide a superior quality to the equivalent MP3 bitrate.
Snap!'s 3 min 42 second Rhythm Is a Dancer (Original Mix), however, comes in at 290kbits encoding (screenshot) on the iTunes Music Store. The file is reported to be encoded with "FAAC 1.24+ (May 7 200...)"
Individual publishers utilize iTunes Producer, an encoding/submitting application which reportedly converts songs to the proper format (128kbit AAC) without any additional options. Whether or not larger labels or Apple themselves have more freedom in encoding options is unknown.
This file may simply be an anomaly, but there have been scattered reports of other songs also offering higher encoding. There does not appear to be any way to distinguish these files before purchasing them.