Its going to be interesting to see what effect this has on last.fm ...It doesnt have appear to have my favorite last.fm feature, scrobbling, but I admit I use last.fm little besides that. Particular, the fact that it ties in to iTunes purchases could be very popular.
That would be a killer feature; I've got 8+ years of listening history in Last.fm, starting with an old Linux jukebox that played music 24/7 (between X11 crashes), and then with iPod and iTunes.
On the other hand, you'd have all these geeks accusing Apple of collecting data for nefarious purposes, just like many of the comments here.
Regardless, Ping looks great and I can't wait to try it out.
