Lot of long winded answers here, plenty of good content. One of my favorite posts on the subject is by Rob Mee of Pivotal Labs, a company known for sustainable work ethics. The full article debunks several myths. Here's what Rob says about >40 hour work weeks.
"Working crazy hours doesn’t get you there faster. In fact, it slows you down. Sure, you can do it for a week. But most start-ups plan to be around for a little longer than that, and developers will going to have to keep programming for months, if not years, to build a successful product. Many start-ups operate as if the pot of gold is just around the corner; if we only work a little harder, we’ll get there. Pretty soon developers burn out, and simply go through the motions of working long hours without any corresponding productivity. Working intensely, for shorter periods of time, is far more effective. Pivotal has helped hundreds of start-ups build systems, and has done it on a strict 40-hour week."