Put "due to" or "because of" together with a supporting cast of words and you have adjectival (in the case of "due to") and adverbial (in the case of "because of") prepositional phrases. That means "due to" should only modify nouns or pronouns and "because of" should modify verbs. Also, "due to" can serve as a subject complement, meaning...oh, the hell with it.
The most succinct explanation I have found comes from Englishplus.com:
Due to means "caused by." It should be used only if it can be substituted with "caused by."
Incorrect: The game was postponed due to rain.
Correct: The game was postponed because of rain.
Correct: The game's postponement was due to rain.
That sounds good enough to me. So let's test it out on this paragraph from yesterday's story in The Province, about Vancouver's annual snow hysteria:
The C52 community shuttle in White Rock was cancelled due to ice and snow on the roads...The West Vancouver Blue Bus also experienced delays due to an accident on the Lion's Gate Bridge.
Using our new litmus test, we can now tell that the second "due to" is correct--it is an adjective modifying the noun "delays" and it can be substituted with "caused by." That first one, however, needs the adverbial "because of" to modify the verb "was cancelled." We can verify this because we know that "was cancelled caused by ice and snow" doesn't work.
Incidentally, the snow is gone today. But there are still plenty of traffic delays due to the ineptitude of Vancouver's city planners.
*Fantods: an ill-defined state of irritablility and distress.