Not being able to run in the background is clearly a problem for the iPhone application, but there are more fundamental issues that really need addressing in all versions of Spotify before it becomes a service not just the hardcore fans will be willing to pay to use.
As great as its playlist system is for promoting collaboration and sharing of lists, as a way to organise a music collection it's distinctly old hat. And, let's face it, when you start paying £9.99 a month for a service, you ultimately want it to be your primary avenue for enjoying music, not just the backup you go to when you don't have what you want in your own collection.
There are one or two other problems you're bound to come up against, too. For instance, efficient though the caching system for offline music is, nowhere can you find out exactly how much space that music is taking up. It's this and the one or two other idiosyncrasies that ultimately lend the iPhone application a slightly 'beta'-like feel, which admittedly is true of the service as a whole. There's also the music library to consider. It's very impressive in scope and variety, but still stops someway short of being comprehensive, and Napster (for one) still has it beaten here.
Thus, while its current shortcomings and limitations are totally forgivable when using the free ad-supported desktop version, when paying £10 a month for mobile access (or ad-free desktop use for that matter) there's still a way to go yet. That isn't to say the mobile app should be free to use, it's ultimately great value if you use it extensively enough, but some work needs to be done before it's practical (and desirable) for the majority to do so.
Anyone who is already totally addicted to Spotify but hasn't subscribed may well find the Spotify iPhone app (or Android counterpart) the nudge they require. However, despite its vast promise, we're not totally convinced that the mobile application and service combined justify a £10 subscription yet. This is another important step in the road, but there's still a long way to go before Spotify fulfils its true potential.