Naturally there is a lot of concern about duplicate content and search engines, there is nothing to worry about. If Google visits your website with a user agent of a Nokia, and it will! It is the right thing to detect that they are a mobile to serve or redirect them to the right content.
They will be more likely to view you as a nice responsible web publisher who thinks of other formats and devices. That has to be a good thing. I have never had any report of ill effect with search engines from anyone using this code.
I use this code myself and rely on organic traffic, it works and I have been told by Sean Owen of Google. Check this
Sean Owen is a senior software engineer in Google's New York office, focused on mobile web-specific search since its inception in late 2004
Sean has suggested headers for this code and told me, without telling me how or spilling as much as I would have liked, that they use a similar method at Google to do this.