I try and keep my lines below 80 columns. The strongest reason is that i often find myself using grep and less to browse my code when working at the command-line. I really don't like how terminals are breaking long source lines (they after all aren't made for that job). Another reason is that i find it looks better if everything fits into the line and isn't broken by the editor. For example having parameters of long function calls nicely aligned below each other and similar stuff.
