I am assuming that the 2.2" PocketDish is the same as the Archos Gmini402? I do not see any Archos devices listed on the Napster To Go site. However, the FAQ at the Archos site states that "The Gmini402 supports MP3, WAV, protected and unprotected WMA(DRM9&10 "To-Go" Music)" and Microsoft's website lists the Gmini402 as Plays for Sure compatible. Also, Amazon's website specifically states that the Archos AV500 (which I assume is pretty much the same as the PocketDish AV500E) is compatible with Napster To Go.
Since it looks like I can get the 2.2" model for $99, I may just have to take a chance.