Just found this map (via Languagehat via Roland.  It is important to note that the different "dialects," the different colored areas on the map, are really different language groups.  The variations within the green area are better referred to as "dialects" of Mandarin.  The other areas are basically disstinct spoken languages.  Which reminds us of both the extraordinary cultural diversity of China historically and its continuing cultural diversity today.  More maps here.