Hi. I've seen this a few times. It even affected Google AdWords. There was a case of some web browsers being unable to click on adverts - so the impression was shown, and the advert was unclickable. Google fixed it, but didn't report back what the problem was, IIRC.
The cases that I've investigated, and indeed, triggered when learning CSS some years ago, involved navigation lists, positioned at the top of the document and then adjusted by right alignment of centering. The problem was, IIRC, one of layers. You can end up with the text visible, but the active part of the link is not accessible through the layer in front. I think I understood the problems better after reading "PositionIsEverything.net". It had a lot of examples of CSS that would and wouldn't work in various situations.
You might also want to look at the practical stuff on A List Apart - they often give reasons and explanations for why vcarious CSS techniques won't work. And I think that Eric Meyer's book on CSS has something about this problem - but I've loaned my copy to someone else and can't verify that.
Cheers, JeremyC.
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