The most important thing is to consider dominant use cases of your web site or application. Do you expect people to use it exclusively on mobile devices? Mobile phones, netbooks, desktops?
Take a look at "Responsive Web Design" by Ethan Marcotte: http://www.alistapart.com/articles/responsive-web-design/
Great article that demonstrates the use of truly fluid layouts using media-queries. Sometimes you need to built out a separate front end for different user-agents, but sometimes media-queries are the perfect tool to service multiple resolutions across different user-agents.
