Satellite radio seems to be gaining a toehold in the US. XM says it has passed the 4 million subscriber mark and was on track to hit its goal of 5.5 million subscribers by the end of the year. “XM said it added 1 million subscribers since late December. XM’s rival in the satellite radio business, the New York-based Sirius Satellite Radio Inc., reported last month that it had 1.4 million subscribers and expected to have 2.7 million by the end of the year.”