What impact will the launch of Spotify’s iPhone and Android versions have on signups? Here’s an immediate sign: “Due to the huge demand in the UK over the last few days since we launched our mobile service, we are going to have to temporarily reinstate our invite system in the UK,” says a post on the company’s blog.That brings the UK back into line with other European countries where you need an invite to sign up for the free version of Spotify, but is presumably an attempt to rein in licensing costs. However, it’s interesting that the launch of the mobile apps – which require a premium subscription – stimulated such a surge in signups to the free service. It’s perhaps no surprise that the launch of Spotify in the US may not happen until 2010. If the availability of the mobile apps spurs so much demand for the free version, the company really will have to hit the ground running on ad sales as soon as it launches there, given the popularity among Americans of iPhone and Android handsets.