The answer depends entirely on what kind of startup you're trying to make and who your audience is. If you want businesspeople and grandparents, having a snappy dot-com is crucial. If they're more tech-savvy, what they remember to punch into Google is a lot more important than the URL. From memory, I believe the domain for Scriptaculous is scripta.culo.us, but I would <i>never</i> type that into an address bar blindly.