A somewhat odd story on WCBS's website this morning on skunked beer:

It flows from taps, is poured from bottles and it's can's can be found in many a fridge. But when you crack a cold one and a funky smell hits you like a ton of bricks, the taste isn't going to be any better. You've been skunked! "A skunked beer is a bad beer. It's a beer that has been sitting around too long, it's been exposed to say oxygen or light and it's turned bad, it tastes bad,” explained, Don Russell.

The story goes on how to find out the date of beer on a number of macro brands. The odd thing is, of course, skunked beer is not bad as it is said to be reversable, if by "skunked" you mean light struck and not just all bad beer. There are many factors that can create instability in beer and light is just one of them. Beer that has been exposed to light is prone to creating off flavours and that smell that is like a skunk's musk:

Beer is sensitive to light, especially in the 350–500 nm range. Light of this wavelength can penetrate clear and green glass and cause nauseous off-flavors in beers bottled in such glass containers and drinking glasses. The beer is said to be “sunstruck” and the aroma and taste referred to as “skunky”. Light instability in beer results from hop components. Hops in brewing have a number of roles: they impart bitterness to beer; provide characteristic hop aromas; suppress growth of ertain microorganisms, particularly gram-positive bacteria; assist in beer foam stability; and contribute polyphenols to the protein–polyphenol complex during wort boiling.

While hops preserve beer they also expose it to this one weakness to light. But unlike other types of instability there are some references in the literature (as smarty pants folks say) that light instability can be corrected by placing it back into the dark and leaving it there for some time. Usually, however, and especially with thoughtless shop keepers, light struck skunkiness is combined with high temperature and bad shelf rotation. The stuff is just old and badly handled.

So while it is true to say that most macro ales suffer over time and will go off, that is not necessarily skunkiness. Lower alcohol beers simply are not built to last as the preservative qualities of the hop acids and alcohol are not present in sufficient quantity to stop the degradation of other organic components. The funniest thing arises, though, when the "best before" attitude is encountered with strong ales. Nothing like finding a shop selling off its bad stock of two-year old Belgian dubbels and tripples. Pure infanticide. You know who your friends are when you get a call to tell you what is happening.