According to Dave Winer, the person who is pretty much the authority on this type of thing (he helped develop enclosures for RSS, as well as RSS itself), today is the third birthday of the podcast.
Dave goes into more detail on his blog, but in essence he notes that while three years ago the exact same tech and terms may not have been there, that series fulfills all the “podcast” requirements (such as MP3 audio via feed enclosure).
He continues to go into more on the Apple squabble over the “podcast” trademark that Apple has begun to defend on a larger scale, noting that podcasts don’t just work on Apple hardware, and perhaps it’d be in Apple’s interest to embrace the podcast term and its widespread use rather than trying to put the term’s users down.
A quick editor’s note: I tend to be a video guy but I’ll try to be fair and cover audio as much as possible. We wouldn’t to forget podcasts!