Mission Critical Applications and Web Based UIs?

Jonny Quest asks: "I work for a company with an excellent Client-Server based product. Unfortunately it's not 'Web-Based' (whatever that means). Ours is a 'Line-of-Business' app, i.e. it means that those people who use it absolutely depend on it for making money - if the app is down, they can't process their orders, service their clients & employees, etc. at all - mission critical in other words, and they spend ALL day in the app. What's the best UI that any Slashdotter has seen for a 'Web Based' app? Something with a rich user interface and some decent multiscreen handling (the current app has about 200 screens...)?" 'Web-Based', that's such a loaded buzzword, and like most buzzwords no one is really quite clear on what it means. Does it mean HTML+CGI? Does that allow for Java? Cold-Fusion? Internet operation or just Intranet (if it's mission critical, it better be the latter, and even then...)? Would any of you recommend deploying a mission-critical application over the web? If so, how would you do it? What technologies would you use and what is to be avoided at all costs?