How are you planning on detecting whether a user is surfing on a mobile device or not? Keep in mind that not all mobile browsers behave like mobile browsers. In fact, the browser on many newer smart phones seem to try to pass themselves off as regular desktop browsers since they already match most features of a full-fledged desktop browser and don't want to force their users to view stripped down versions of websites designed for more primitive phones.
Even mobile resolutions are catching up to desktops and laptops with WVGA (800x480)—and not to mention netbooks and tablets (will they be considered mobile devices and prohibited from viewing the main site?).
Whatever you decide to do, be careful you're not creating a situation common in the IE4 days where non-IE browsers were being turned away from sites because their browser was "unsupported" (some sites were so arrogant as to recommend that the user "upgrade to IE") even though the user's browser was still fully capable of rendering the page perfectly fine. In most cases, it's almost always better to give an unrecognized browser the benefit of the doubt and let the user decide whether the site adequately supports their browser setup or not. If everything is out of alignment and the navigation is broken, then the user will leave the page. So there's no need to turn the user away off-hand.
Personally, I'd let any user view the standard site if they wish, but just flash a warning when a user is suspected of being on a mobile device, letting them know that the current site isn't designed for mobile users, and a specialized mobile site is in the works.
If you really need to reject users, I would at least put up some kind of "subscribe to updates" box to allow mobile users to be notified when the mobile site is launched. This way you know how many mobile users are interested in the site, and mobile users won't feel like they're being left in the cold.