Don't do it.
Linking the logo to the front page has become a de facto standard. Linking it to the contact page would only confuse users who expect to go to the front page, while users who want to go to the contact page would probably not click the logo.
Update in response to the comment below:
In my opinion, a language selection front page is a bad idea. Remember that people come to your site in order to accomplish a task. That task is never to choose a language. Also, consider how you want Google to see your front page.
A better alternative is to serve the front page in a language determined y the Drupal Language Negotiation API and make it really easy for users to switch languages if they want to. Read more about multilingual front pages at HowTo: Different home page (default front page) for each language and Drupal 7: Cracking the multilingual front page nut.
Now, if you still want to change the url the logo and site name link to, enable the
THEMENAME_preprocess_page function in your Zen sub theme (you did create a sub theme, right?) and alter value of