While it is desperate for citations, it looks like the Wikipedia article supports that: http://en.wikipedia.org/wiki/Dialect#Dialect_or_language
There is also an episode of one of my favorite podcasts (Slate's Lexicon Valley) that focuses on this issue, particularly in regards to what has sometimes been called "Ebonics": http://www.slate.com/articles/podcasts/lexicon_valley/2012/0...
But in the end, I really believe that if it weren't for the common written language, no one would ever think of Cantonese as being just a dialect of the same language as Mandarin - the differences between the two spoken languages are so massive.
I personally prefer to think of Chinese as a language family. It would be a much larger, and very interesting, topic. For the discussion on this thread, I use "dialect" other than "separate languages", since Cantonese, We Chinese, etc. are described as dialects in most materials I read.
Where it gets confusing is when nationalism gets involved. Mandarin and Wu are different "dialects" of the same "language" because they're from the same country. Italian and Spanish are different "languages" because they're from different countries. It's a political distinction.
Linguists tend toward using mutual intelligibility as the difference between a language and a dialect, but it's fuzzy. Beijing and Shanghai dialect are almost totally mutually unintelligible, but if you walked from Beijing down to Shanghai, each village, town, or city you pass will speak a dialect that's still mutually intelligible with the immediately neighboring village/town/city's.
But really, the words "dialect" and "language" mean essentially the same thing in English.
So, its a "ring language" (by analogy to a "ring species" [1] in biology.)