Seems like America's doors are still wide open to new musical immigrants, because Spotify's stateside foray has just boosted its overall valuation to $1.1 billion. That's based on its two most recent investment rounds, which racked up another $100 million from new and old investors who are clearly convinced that the streaming platform can carve out a niche between existing rivals like Rhapsody and new entrants like Google Music and iCloud. Meanwhile, AllThingsD are reporting figures from an unnamed source who says that the London-based company now has 1.4 million US subscribers, of which 175,000 are coughing up $10 per month for premium features like mobile access. If accurate, that equates to a conversion rate of 12.5-percent -- not quite as high as the 15-percent it achieved in Zamunda Europe, but still impressive.