Every now and then, we're asked by a client if they should have a separate version of their website for mobile devices. These are generally called 'mobi' sites, because they often have a '.mobi' top-level domain name instead of the common '.com' or '.org'.
The upcoming standards of HTML5 and CSS3 will provide better support for mobile devices with their Media Query properties. Unfortunately, no browser fully supports HTML5 or CSS3 right now. It'll take at least another year to eighteen months for the major browsers to support these new standards. And by then, the need for them will be greatly diminished by having mobile devices with better screen resolutions.
So where does that leave us from a design standpoint? None of the current generation of smartphones can handle flash (not just the iPhone as seems to be common wisdom). So as long as your site isn't flash-heavy, it should display fine on the current generation of smart phones. There may be a small group of users that don't have an adequate phone to view your website, but they will rapidly diminish over the next 18 months.