Here are the two definitions of the 大辞泉. Since there are not information about the context, you'll hace to find which one suits better.
額当て（ひたいあて）
１．冠の額に当てこじの後部で結ぶひも。抹金（まっこう）＞＞＞古代、冠のヘリに紅の絹で鉢巻をして後ろで結んだもの。下級の武官が用いた。もこう。
2．額金に同じ　＞＞＞　武士が軍用の鉢巻の額の部分に入れた薄い銅や鉄の薄板。額当て。
In the first definition, 額当て designates the headband made of silk that was wrapped around the edge of the headgear (to fix it) and tied in the back. It was used by lower rank officers.
In the second definition, 額当て designates the thin board of copper or iron fixed on the forehead part of the headband described in the 1st definition.

If you have ever seen Naruto (see link), it will give you an idea of the headband looks like with a thin band of metal on the forehead. His 額当て is even for sale on rakuten! (second link)

Hope this helps

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Note added at 4 heures (2008-12-31 08:07:23 GMT)
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to make it short, 額当て designates "headband" OR "thin board of metal fixed on the headband"

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Note added at 4 heures (2008-12-31 08:17:41 GMT)
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Ok, now I've been reading more about Naruto in English and I found the expression "plate headband" quite a lot. I think it could be what you're looking for.
http://www.stylinonline.com/headband-naruto-plate-red.html

Ok, I didn't find how to modify target word in my first answer, so I have to open a new one. The explaination in my first answer is still valid, that's why I don't delete it.
Now I've been reading more about Naruto in English and I found the expression "plate headband" quite a lot. I think it could be what you're looking for.


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Note added at 4 heures (2008-12-31 08:24:17 GMT)
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ps : some call it sweatband but that expression is rather appropriate for 鉢巻、not 額当て