The market penetration won't be particularly high as long as it remains on one device, but there has been some indication that it is coming to other mobile devices. Spofity apparently posted an advert for a developer for Nokia's s60 platform, so there is a good chance a similar application will be on its way for high-end Nokia phones. It remains to be seen whether they will manage to get an iPhone application through Apple's fortress. I can imagine Apple coming up with any excuse to block it from the app store.
In any event, it certainly represents a threat to iTunes. It will probably also help Spotify sell more Premium subscriptions, but there has not been any information yet about what the pricing will be for the mobile app. It will almost certainly be data-intensive, so if its being used on the move the mobile operators will want a piece of the action.
Also interesting it the 'offline sync' feature which raises copyright and licensing issues. Details are few and far between, but it seems like something of a download/streaming hybrid, being neither download-to-own nor streaming in the conventional sense.