Nielsen//NetRatings have just put out some interesting stats which suggest that nearly 7% of US adult internet users have consumed an audio podcast in the past 30 days, with 4% downloading video podcasts during the same period. That's an audience of 9.2m for audio and 5.6m for video - not huge audiences, but the browser analysis included here suggests that they are above average in tech savvy, which may make them more lucrative for some advertisers, I suppose. On a separate but interesting note, check the monthly update on advertising spends at the end of the memo. Some things never change (Vonage number 2 at $37.5m in June - hey isn't that 4% of your current market cap?) while some certainly do (Skype outspending TimeWarner? Sacrilege.)
UPDATE: It's obviously not just a US phenomenon, as attested to by Neil over at ThamesValleyPod. He just pinged me to say that total hits in June were 81,500, but that this has risen to over 133,000 in the first eleven days of July.