I find it odd, this practice of renewing a show after a successful pilot premiere (or after only a few episodes). That's not to say the show may decrease in popularity, but that's always a possibility. Maybe people tuned into the premiere for curiosity, or a particular star, or whatever, but that doesn't mean repeat viewers week after week.
A lot of cable channels, and some networks (see Fox re: two-year renewal of Glee) have gotten into this practice more and more. HBO is the newest example.
Boardwalk Empire, HBO's newest show, premiered to decent ratings for the channel on Sunday night. 4.9 million people tuned in during its premiere at 9:00 p.m. on Sunday night (with 7.1 million viewers watching the premiere and repeat episodes. That's HBO's largest premiere since Deadwood premiered back in 2004 (which had The Sopranos as a lead-in).
HBO was thrilled with the ratings and has announced that the show has already been renewed for a second season:
“All the ingredients aligned for this one, from Mark Wahlberg and Steve Levinson’s initial pitch, to Martin Scorsese’s enormous contributions as director and executive producer, to the genius of Terry Winter and the expertise of Tim Van Patten, to a stellar cast led by Steve Buscemi,” said HBO programming president Michael Lombardo. “The response from the media and our viewers has been nothing short of amazing.”
It will be interesting to see how the ratings continue over the course of Boardwalk Empire's first season and if the renewal turns out to be justified. It will also be interesting to see if this practice continues and whether more networks engage it in down the line.
HBO did this same thing for Treme as well.