If you don't mind a bit of extra work, you can use media queries to serve a different stylesheet to larger devices.
If you want to have just one mobile stylesheet, I would forget about designing for a width altogether. Just give a linear page, flowing from top to bottom. That will work on any CSS-capable device without having to worry about the screen size. If people don't like that, you can give them the option to switch to the "regular" site and have the device scale it and adjust it as necessary.
Personally, I am quite happy to use "regular" sites on my smartphone (basic Nokia), as long as they don't go overboard with scripts, fancy-pants styling and large images etc, and allow the phone to adjust the design as I go. I often find this easier to work with, especially for sites that I'm used to using on a PC, because then I know exactly where everything is, and don't have to worry about the site being different.
(That's my own view, and may or may not be shared by other people...)