Not true. When you are streaming a NOW channel then obviously that will consume a lot of the bandwidth (3-4MBit/s or so). But when you're not streaming a channel pretty much the whole bandwidth is available to the internet. I regularly get 6MBit/s downloads out of my 6Mbit/s connection.
Based on that what should I expect when I order a 3M connection and NOW TV? Will the TV service cripple the internet connection?