Spotify Promises Nearly Perfect On Demand Music Service

"Some quick background: the promise of Spotify is music, on-demand, from any computer with an Internet connection. Which sounds a lot like Rhapsody, Napster, Microsoft's Zune Pass, or any other of the countless subscription-based services that have come and gone...except that there's a free ad-supported version. Sort of like what Qtrax was promising, only with streams instead of downloads (and actually available instead of merely promised). Or perhaps sort of like Imeem, only without the confusing attention-deficit-disorder interface. Or--let's be honest here--like all the dearly departed P2P services of yesteryear, only legal with partners including the four major labels plus independent digital distributors CD Baby and The Orchard."

Certainly attractive, but I'd really like to see some sort of off-line mode that would work with an MP3 player.  This would be ideal for work, but would violate a lot of employer's Internet usage policy.  Would be great fun for home use and for making the perfect mix.