contextがないので本当に慣用句として使われているのか不明ですが、ネットで検索したところ、いくつかヒットしました。

--------------------------------------------------
Note added at 2 hrs (2007-05-09 12:30:29 GMT)
--------------------------------------------------

exhibited that they had large bending strength, adhesiveness to dentin, small water absorption and expansion, and low rate of decomposition

余計なtextが入ってしまいました。上記サイトをご確認ください。

In this context, I think what we are looking at is the biggest obstacle to overcome, which, in this case, is cost. We have a joke that says, "Where does a 500-pound gorilla sit?" The answer is "Anywhere he wants to." It's difficult to do anything with a gorilla, so what they're saying is that the cost is high and will be hard to deal with.

Please note, the term you have is incomplete, as the real term is

"The 800-pound gorilla in the room."

You can find loads of examples on google.

This is a phrase that describes a dominating, controlling factor in a situation, or something with an overbearing presence.



--------------------------------------------------
Note added at 15 hrs (2007-05-10 01:31:59 GMT)
--------------------------------------------------

I should add, as Satoshi pointed out, an "800 pound gorilla in the room" typically describes something you wish to avoid, but is simply unavoidable.

There is also another similar phrase: "the (big) white elephant in the room." This has a very similar meaning the "the 800 pound gorilla," but in my impression, tends to describes topics that are very obvious, but also difficult to discuss out in the open. An example would be:

"Hu Jintao's recent visit to Japan highlighted economic and environmental agreements between the two countries, but the pending issue of comfort women and Japan's wartime history was really the big white elephant in the room."

○目の上の瘤(ｺﾌﾞ)
自分よりも地位や実力が上で、とかく自分の活動に邪魔になるもののたとえ。「目の上のたんこぶ」とも
広辞苑