I must respectfully disagree with stanis; if a poor person on the street were to ask for money, forgetting about this person would be utilitarian only if one were actually poorer than this person. If one were substantially more affluent than this person, giving him money would cause less harm to oneself than good to him, and therefore would be the utilitarian thing to do. Such is the principle of utilitarianism: the greatest good for the greatest number.

The problems with this are twofold:
1) some external philosophy and/or theology must be used to determine what is "good", and
2) it's more or less impossible to account for every good or bad effect, and its even harder to quantify these effects.

Thus, the idea of some that utilitarianism can be used to justify actions that "commonsense morality" dictates are wrong. Misused, it can be. But it can also help in some very hairy ethical situations.