Easy as it seems, it escaped me for almost 30 minutes. Embedding a driving directions form on a website is generally quite simple. You just need to imitate the form that Google or Yahoo uses themselves to get the data.
Yahoo is actually quite simple. All you have to do is post the form to http://maps.yahoo.com/# with these two form fields: q1 and q2. I generally have q2 be a hidden field that has the destination address.