"I'm not sure why people are giving answers about how to change the DPI setting for X11. Yes, Mac OS X includes a port of the X Window System, but I'm assuming since you didn't mention it in your question, it's not what you're asking about.
Unfortunately, the real answer to your question probably won't make you happy. Resolution independence has been ""coming soon"" to Mac OS X for years. Here's how it looked in Mac OS X 10.4 Tiger in 2005, and here's how things stand now in Mac OS X 10.5 Leopard. And not to talk out of school, but things aren't looking much better in the upcoming Mac OS X 10.6 Snow Leopard.
All of that said, if you want to give it a try, install the Mac OS X developer tools (they're on the Mac OS X installation DVD) and then launch the Quartz Debug application that you'll find in a subfolder of the Developer folder on the top level of your hard drive. Then select User Interface Resolution from the Tools menu and you'll see a window like this:
Move the slider and see just how awful everything looks. Note: you'll have to re-launch most applications before they'll use the new DPI setting. (The Dock and a few other things auto-re-launch when you move the slider.) Overall, I think you'll be disappointed. The wait continues..."