Spotify Premium will work offline on laptops later today 01/10/09 - 14.49
As with the iPhone 3,333 tracks can be cached offline. It’ll work on up to 3 computers at once, playing on one is said to pause the others – but obviously not if you’re offline. This is premium customers only.
When I did my Spotify Premium review this is one of the features I asked for, and already it’s here.