I don't know if it's a general Java problem or just Minecraft, but the U.S. International Keyboard mapping is not fully supported. If I write in the chat/console some letters using dead keys, that doesn't always work depending on the dead key. I guess the problem comes from the fact that the U.S. Int. adds some weird dead keys to deal with some accents.

Regular ones works :

ê : combination of dead key ^ then e
ñ : combination of dead key ~ then n

Some non « conventional » doesn't work

é : combination of dead key ' then e : Minecraft result : 'é
è : combination of dead key ` then e : Minecraft result : `è

But other non « conventional » works :

ë : combination of dead key " then e : Minecraft result : ë

These a just a few examples

But I bet that's a Java issue I haven't try on Windows however.

Update:
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With previous versions but at least with launcher 1.3.4 and Minecraft 1.7.2, the dead keys are just not registered at all.

Uptate 2:
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It seems that every keyboard mapping with dead keys is affected.

Uptate 3:
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The issue is for any Keyboard mapping using dead keys.