Like others have said, replace is different from be the same as. Headphones can replace speakers, but they will never be the same as speakers. Personally I would never replace my speakers, the main reason (as I suspect many people also feel) being bass. With headphones, bass only goes to your ears/the side of your head. Speakers can rumble your entire body - its a much more powerful and engulfing experience - one that headphones simply cannot achieve. Let alone the fact that no current headphone can simulate soundstage like speakers can at the moment, but I think that given time that is, at least theoretically, possible.
edit: I suppose if you could get an external sub hooked up in sync with your headphones you might be able to achieve a speakerlike feel, but I doubt it would work quite right.