I had 3 roommates at one point, and all 4 of us were subscribe to Netflix. It was VERY easy for us to hit the cap with everyone watching Netflix alone, let alone playing games or streaming music in the common areas.
After I moved out and got married, my wife and kid routinely have Netflix playing in the background as they sit at home during the day while I'm at work. When I go home, I usually have work to do at home as well, which only adds to the usage.
Using numbers found in the article, if you use Netflix on a daily basis for 3 hours a day watching HD content purely, it would park you at about 210GB of the 250GB cap according to Netflix themselves:
http://blog.netflix.com/2011/03/netflix-lowers-data-usage-by...
So yes, using Netflix and Pandora can easily put you over the cap.
EDIT (for clarity): This article talks about the quality settings in the account settings area, but if you adjust the settings to allow for highest resolution, it would still apply.