Great entry from Khymos last week. Isohumulone is a compound found in beer imparting some of the (good) bitterness. Unfortunately, the magic of photochemistry can wreak havoc on it.
Isohumulone can be excited by the agency of riboflavin, an endogenous chromophore, causing it to cleave into a pair of radicals, one of which can react with free cysteine radicals in proteins, yielding a low molecular weight thiol, which can give beer that "skunky" taste. Dark bottles prevent this, but as the entry points out, clear-bottled beers are far from immune.
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The beer found in clear bottles is generally not susceptible to skunking, because the brewers use hydrogenated hop extracts.
What you say is true for the beers generally found in green bottles, though.
I recently posted a proposed mechanism for the isomerization of humulone to isohumulone during the boiling process of beer brewing. http://homebrewandchemistry.blogspot.com/2007/02/hop-chemistry.html
Sorry for the shameless promotion.