[wmf]

Nothing can keep the RIAA off you; if they want to sue, they'll sue.

The main difference between lending CDs and "lending" digital files is that copies are being made in the latter case, and The Man will say that those copies are not authorized, are not necessary for playback, and are not fair use. The Man also sometimes takes the position that an individual cannot "outsource" their fair use rights, so even if it's legal for you to personally make a copy in some situation, they claim it's not legal for you to authorize a third party (especially a for-profit company, e.g. a music locker) to make that same copy on your behalf. So some things that might be legal in a P2P implementation (as long as it's not called "P2P", they hate that) would be seen as illegal in a centralized Web 2.0 implementation, and since it's so hard to make money from P2P people just don't bother.