This issue came up at http://stats.stackexchange.com/q/3474/919 , for instance. Let's look at the question constructively: how can we use homework questions to the benefit of this community? These include:
A good homework question can be a good question, period. What does it matter if it is homework?
Often, the elementary nature of homework questions draws multiple different answers, which illustrates relationships among fundamental techniques and exposes the foundations of statistical theory.
A homework question is usually fairly specific (at least it can be), whereas questions arising in other circumstances frequently need considerable clarification before they can effectively be answered.
The possibility, then, that homework questions can contribute to the site suggests they be allowed. Furthermore,
- If homework questions are not explicitly allowed, people will just be forced to disguise their homework questions, which typically makes them vaguer and requires more work to answer. (We now have a "homework" tag. Let's encourage its use.)
Concerning how to answer homework questions: I would beg this community, both collectively and individually, not to use "contempt" and "sadism" as a tool for anything. That only leads to ill feelings, flame wars, and flagged posts. We should be especially alert for the possibility that a badly constructed question (that perhaps looks like homework) is due to the questioner's unfamiliarity with English. We should respect such questions more than most due to the effort required to formulate them. (If you're not convinced of this, sit down for five minutes and write out a question about statistics in any language you are not fluent in. Now think about posting this where the world can see it and potentially having to engage in a follow-on dialog in that language. How does it feel?) Accordingly, it is foolhardy and counterproductive to rush in with contemptuous or thinly veiled suggestions. It's easy to get people to visit a site once but hard to get them to keep visiting it; a policy of gracious and courteous answers from the community will IMO promote return visits.
Nobody wants to spend their precious spare time just doing someone else's homework for them: that benefits neither party. An approach frequently taken on the math site is to provide hints, a solution strategy, or a sketch of a solution (cgillespie's option 2). Sometimes these get amplified into a full-blown solution in response to comments, but even then some respondents have modified the problem slightly (e.g., to use different constants in an expression) so that some work has to be done to answer the original question. That strikes me as a good balance that addresses the positive aspects of answering homework while retaining the utility of the problem in helping the questioner learn.
Finally, why don't we build consensus on this issue and get it expressed in the FAQ?