"It reported 483,075 subscribers as of March 31, a 39 percent increase from the fourth quarter and six times the subscribers from the first quarter of 2002. In April, XM announced that it had surpassed the 500,000-subscriber level." http://biz.yahoo.com/ap/030508/earns_xm_satellite_2.html
I think this should be interpreted as "sometime in April, they said..."
This is how many subscribers they've added in the last several quarters: 135,916 145,605 64,836 60,476 48,509 27,233
Stock does seem kinda pricey but I haven't looked at it that closely (and besides, who am I to say? <g>)