
If you read the last couple of books in the series closely (well, #3 and #4
at least), there are at least two points at which the real Question is
implied.  Conversations proceed much like:

Zaphod: What *is* the ultimate Question, I wonder?
Arthur (not paying much attention to Zaphod, but needing a random seed for
  the Infinite Improbability Drive): Think of a number, any number.

Actually, it may be Marvin who uses this phrase a few times as well, and
everytime it's arranged such that "Think of a number, any number" could be
an answer to someone's question about the Question.

I kind of like it.  Very mystifying.  It's not even "pick a number" or
"tell me a number", just "think of one".

