This seems like bad news for Apple, to say the least. A few Chinese websites are now selling $200 gift certficates to iTunes for less than a few bucks, which means that it's likely hackers have figured out the algorithm to determine gift codes on Apple's music store. As with most online codes, iTunes gift certificate numbers are generated by a formula somewhere -- figure out the formula, and you can generate your own codes (though it's of course tough to do and highly illegal).
The good news is that this might be an easy fix for Apple: they'll just have to re-figure the formula. The tougher thing to do will be to determine which of the old codes to honor -- they'll want to make sure to approve all of the cards on the shelves at Best Buy right now, while still trying to catch all of the illegal codes generated by hackers.
But then again, we're talking about a digital store that's already making cash hand over fist. Maybe even if one hacker on a shady website has figured out how to generate iTunes codes, Apple isn't too concerned about losing a few thousand dollars when they're still selling millions of dollars worth of music and content legitimately.