Philosopher SteveG can hardly be blamed for the straw man he presents of psychological egoism: This straw man has been stuffed, mounted and draped with finest attire by the most eminent of the philosophical canon, many of them, sadly, proponents of psychological egoism itself. Still, I'm irked that a person of his obvious intelligence just takes another whack at the scarecrow rather than searching for the equivocation at the heart of the controversy.
The equivocation is, of course, that the "self-interest" which is contrasted to "moral behavior" represents a different concept than the "self-interest" which is contrasted to "altruism".
I don't know why philosophers in general seem so confused about psychological egoism. It's pretty straightforward science to describe conscious human behavior--precisely the sort of behavior to which we apply our philosophical notions of morality--as the outcome of teleological goal-setting mental states which devise strategies to actualize the subjectively preferred outcome from the foreseeable choices.
[In English, please!]
People imagine what might happen in the future, figure out what they want to happen from what can happen, and do what it takes to (hopefully) make it happen.
This causal notion of self-interest seems like an obvious match for the term "psychological egoism".
Psychological egoism is not rocket science, not even brain surgery, but it is scientific. It's logically possible that our brains might not work in this manner, and we could distinguish these cases indirectly by observation. Many computer programs, for instance, do sophisticated tasks without any internal notion at any level of abstraction that even remotely resembles goal seeking to subjective preferences.
The notion that "altruism" must necessarily entail some violation of this causal mechanism seems ridiculous. If I am interested, explicitly and consciously, in the well-being of another person; if their subjective happiness fulfills my own (causal) self-interest, why cast suspicion on my altruism? We can simply contrast altruism, being interested in others' subjective happiness, with selfishness, being uninterested in others' subjective happiness.
Of course, the problem with this trivially obvious analysis is that it does destroy the notion of an objectively true morality, forcing moral philosophers around the world to start cramming statistics in hopes of admission to the psychology department.
The only way a human mind, which operates in this goal-seeking, preference-satisfying manner, would adopt any moral principles would be as a part of a long-term strategy to optimize preexisting subjective preferences. The only reason I need to encouraging you to be "altruistic", that is concerned with others' (i.e. my) well-being, is because it's an effective strategy for satisfying my subjective preferences. The only reason for me to be altruistic in return is because that encourages you to be altruistic, to our mutual benefit.
The "objective truth" of such a morality would be entirely irrelevant, even if there were even a hint of an epistemic basis for such objective truth, which there isn't.