It would be pretty illegal if you could swap songs with anyone you like.
Microsoft tried it and came up with "Squirting", which is crippled enough to be completely useless.
No officially distributed third-party application will have access to the iPod's music player functionality for this exact reason, so count 100% on it NOT happening (because Apple would rather go bankrupt that implement a half-broken function as brain-dead as the Zune's "Squirting").