We finally decided to create a mobile page for our site Our website is an old version (5.0) of drupal and we don't want to upgrade the theme (we did this before with another site and all rankings were gone for more than a year). So we created a page on a subdomain (mobile.oursite.com) and this page is presented to users who come form a mobile browser.
So basically, mobile users click on the link in search but instead of landing on website's page, they are redirected to a mobile subdomain.
Your thinking is in the right direction. As long as you treat googlebot the same as any desktop user-agent (and googlebot mobile the same as any mobile user-agent) you are essentially in the clear on the redirect.