He probably is referring to heat-issues on a netbook depending certain design - constraints. Before buying a netbook for using it as a router I would check if certain air-vents are covered when the lid is closed. Some Net/Notebooks have an air-cooling system which is designed in such a way that the air-vents are more or less closed when the lid is closed as the manufacture didn't assume that the laptop would be ran with the lid closed. Those are obviously a bad choice as in the long run the lifespan would be decreased.
However, I am not quite sure if you can use a netbook as a router as most netbooks (at least all I can think of the top of my head) only have one ethernet port. As far as I know most linux/unix-y-router distros: ipcop, smoothwall, pfsense require at least 2 ethernet interfaces: one for wan, another one for lan.
May I suggest that instead of a netbook you might be better of using a Intel Atom D Mainboard (like: http://www.newegg.com/Product/Product.aspx?Item=N82E16813121442) and installing another NIC. The power usage should be quite simliar compared to a netbook as the cpu and chipset are more or less the same. Hope this helps