By GTA standards, CW's opening week was modest to say the least. Many are already calling the game a sales failure in relation to the size of the DS userbase. However, I think a couple of things are often ignored in the thought process that reaches such a conclusion:
1) GTA is very much targeted to a male audience. I'd be surprised if females accounted for as much as 5% of GTA IV's sales. The DS userbase is around 50% female.
2) CW is an 18-rated game. I really don't know what the average age DS owners is, but school kids undoubtedly account for a large proportion of them.
If the target audience for a GTA game is males who are over 18, people shouldn't be looking for huge numbers from CW. The target audience may account for less than a quarter of DS owners.