Sharply pointed

An exact antonym might be "necessary," simply because a pointless endeavor is one in which there is no reason for it to occur, while a necessary endeavor is the exact opposite. Contrasting examples of these might be moshing at a rock concert (pointless, really), and having to have your head stitched up after falling to the concrete floor while moshing at a rock concert (necessary, unless one enjoys concussions).
Of course, if one wants to be silly, one could say that the antonym of "pointless" is "sharpened"--as in pencils.

Aaron

How about "purposeful"?

As the two previous respondents have illustrated, "pointless" is used in distinct ways. The first respondent suggested the antonym "necessary"; in this case, "pointless" is a synonym for "accidental" or "random". The second respondent suggested an antonym of "purposeful"; in this case, "pointless" is a synonym for "random" again, but with reference to human motivations rather than philosophical necessity. A third antonym might be "meaningful".

A general thought strikes me that to find antonyms, start with synonyms.

Matt,

Your insights seem very pointful to me.

Rus

Depending on the connotations in the use of "pointless" appropriate antonyms could be:
relevant, incisive, forceful, or brilliant.

It strikes me that the opposite of pointless, when used as in "pointless question", is "pointed". Which makes perfect sense.

In belated recognition of Michael Fischer's yeoman service to the language over the summer (not to mention his continuing presence in this conference, and his forbearance from dragging in any more poems by Robert Bly), I'd like to present him with the Word Fugitives Bureau of Investigation Special Agent of the Week award. Michael, a choice of splendiferous prizes will be offered you privately, by e-mail, in order to preserve the sense of mystery surrounding this august award. Three cheers for Michael! Hip hip hooray!