Everyone knows the words used in Diceware passwords (all 6^5 = 7776 words are published) -- they're all common words. Everyone seems to know that we're not supposed to use dictionary words for passwords because the "dictionary attack" can rapidly guess a single dictionary word. So it seems reasonable to leap to the conclusion that a dictionary attack can also guess a Diceware passphrase pretty quickly.
Can a dictionary attack mounted now (2012) crack a Diceware passphrase before 2033?
In particular, is the claim on the Diceware page "A seven word pass phrase is thought to make attacks on your passphrase infeasible through 2033." accurate?
Is that still true even if the attacker knows that I always use Diceware passphrases, and knows which language I use?
How does a five-word Diceware passphrase compare to the common recommendation of 9 "completely random-looking gibberish" characters?
(I'm asking a very specific question about the recommendations on the Diceware page, since related questions passphrases - lowercase and dictionary words and XKCD #936: Short complex password, or long dictionary passphrase? seem to get sidetracked onto things that are not really Diceware passphrases).