Mining consumes very little bandwidth.
Running a bitcoin P2P node consumes a noticeable amount of bandwidth -- my VPS ran out of its 1000GB allocation this month.
Do you have an idea what part of the protocol munched up most of the bandwidth? It's a definite concern for people who want to run seed nodes to stabilize the network, as well as people interested in a mobile client.
If I had to guess, would it be exchanging/downloading the block chain?