Take a look at the home page for the Dr. Adsense Podcast.
Now, the Podcast is already in all the engines (iTunes, Yahoo, etc.) and you obviously don't have to come to the site (or register) to listen to the content. We even but the subscribe buttons on the bottom of the page. Very subtle subscription scheme – to say the least.
However, guess which # is bigger – yesterday's subscribers (according to Feedburner) or yesterday's mailing list size?
The mailing list is bigger – by almost 170%.
Joel just released version 3 of his Adsense ebook (an amazing read, but that's another posting ;-)). I just did a mailing to that list. I bet it will do very well.
I'll report the numbers in a few days.