Not to mention, the optimal width for reading prose is somewhere around 60 characters per line. The eyes don't need to track so far back when they reach end of the line and so on. (I'm sure there's research on this I can't bother to conjure up right now.) (See also: newspapers, magazines. They break their text into columns, too.)
Usually when I need to print text, I print two pages per sheet (A4) just to get the line lengths to something satisfactory. It also tends to save a bit of paper...
Anyway, code. My stance is that you can't achieve clean code unless you keep your line length under 80 characters. The widescreen monitor argument? Either put multiple code listings side by side, or pivot your monitor. 10:16 or 9:16 doesn't seem so wide anymore, does it?
(...those cheap monitors that use TN panels don't pivot so well...)