Other subscription services like Napster and Rhapsody already let you stream 25 free on-demand songs a month, but it seems as though the Slacker service wouldn't apply a cap. You'll also be able to create a Slacker station as specific as playing only two artists.
There's no word on pricing yet, though Slacker already has an ad-free model that costs $5 a month, and presumably this would be an added premium on top of that. But indications are that it may be flexible:
If its subscribers mostly continue using the interactive radio features rather than playing on-demand music all day, Slacker might be able to offer them a lower-priced or more sustainable service than services that rely almost exclusively on expensive-to-license on-demand playback.
Also unclear right now: timing. But whenever it launches—if the price is within reason—this could turn out to be a pretty great service. [Wired]