I don't think they can, although I have only limited experience. The highest end in-ears I own or owned are the Sennheiser ie7, ie8, ue triple fi and super fi 5 pro. All of them in the price range between 150 and 250 Euros (where I live). They all do sound good.
Compared to an RS-1 out of an iPod or my Sennheiser HD 650 out of an Earmax amplifier there is only one conclusion: sound wise, those in-ears have not the slightest chance against a real headphone. The RS-1 and the HD 650 cost between 2 and 3 times as much as the mentioned in-ears.
I cannot say how a 200 $/Euro headphone would compare to an ie8 or triple-fi though. But high-end in-ear vs. high-end full size headphone is a homerun for the headphones.
Despite of that, when I am in the underground train or a bus or walking, I prefer in-ears. They help damping ambient noise and allow to listen at reduced volumes. If you don't want tinnitus or become deaf some day, I recommend in-ears for all situations except at home. On the go all the quality a full size headphone can deliver does not help anyway.
Edited by mironathetin - 6/8/11 at 12:09pm