Comment Re:xkcd (Score 3, Interesting) 175
Really, Slashdot? 4, Insightful for a comment that has no idea what it's talking about? All you need to do is read the Wikipedia article you link to:
Generally, dictionary attacks succeed because many people have a tendency to choose passwords which are short (7 characters or fewer), single words found in dictionaries or simple, easily-predicted variations on words, such as appending a digit.
Emphasis mine.
A dictionary attack is a fast way to crack a password consisting of a single word. The conventional wisdom of how to thwart a dictionary attack is to replace letters with symbols and append a few more symbols to the end. Randall's comic is intended to establish that simply using more than one word will thwart a dictionary attack much more effectively.
His comic does this by calculating entropy. His estimates of "3 days" and "550 years" are the theoretical best time to crack the password, and already take into account that English words have lower entropy than their constituent characters. Actual attacks such as dictionary attacks are slower than these theoretical best estimates.