thats ok - I'm not worried about disk i/o - although if I was using a SSD based netbook, that might be an issue. When travelling, I use a "wireless broadband" link - as they call them here in Australia - internet via mobile phone. Mine has a 1Gb per month limit, adequate for getting emails on the road, but not heavy p2p usage! Just wanted to be sure that using bitcoin on that link would not max me out in a few days!
Note that the statistics theymos gave can and in my opinion mostly is just keepalive messages and not actual block sharing data.
If you turn on Bitcoin every 24 hours or so? I don't think it should max out your datalimit.
Then again, only way to find out is to try it!