It is a slippery slope my friends, that's why I recommend FREEDOM, a free downloadable application for your Mac that let's you put a self-imposed ban on using the internet (or IM, etc.) for any period of time up to 8 hours. The only way around this ban? Reboot your computer. Not the toughest by-pass, but enough of a pain that it should limit your online wandering.
Check it out here.