It seems scammers have found other ways to trick people into subscribing to Premium SMS. This time they are spreading Mac OS X fake installers that ask people to give their mobile numbers to "register" the software.
This is obviously an evolution over those web sites promising "compatibility tests", "daily jokes", "daily horoscopes", "you just won an i[device]" and other tricks.
Interesting to note that some of the installers don't have to actually install anything, as they already got the mobile number. Some do install something, but usually an app that's already available for free elsewhere anyway.
Be aware of scammers.