Is your ISP wanadoo by any chance?
If it is they state in there webpages that they will not stop your service if you go over your cap but if you continually exceed it they wll ask you to upgrade to the next one. Also I believe wanadoo only started to enforce there caps a short while ago.
Online gaming uses alot of bandwith and so does downloading music tracks so it is quite feesable that your usage could be very high. You can get a netmeter from click here which will help you to monitor the ammount of bandwith you are using, you can set it to load at windows start up and then it will sit in the system tray.
At least your ISP has just asked you to either cut down your usage or upgrade to the next level if you were with tiscali they would charge you £1.50 per gig or part off that you had used above your usage cap.
I guess that yes ISP could notify you when you are reaching the limit of your cap but I think the main reason they exsit is so that ISPs can offer broadband on a price that is about the same as dialup and therefore attract more customers.