I don't have any hard numbers either but I agree with a lot of what Dan said.
A few years ago podcasting directories did bring in some listeners for
my podcasts but not so much anymore. People now seem more likely to
do simple Google searches or look for iTunes recommendations of
similar podcasts when they do iTunes searches. All simple word of
mouth works very well. I'm involved in many online communities for
the topics I cover and my email signature and my forum signatures
include links to my podcasts which brings in folks.
I also agree that having a nice flash web player on your website/blog
is very important since many of our listeners say they listen while at
work so they simply stream the episodes that way.
Fan of Science Fiction or Fantasy?
Then you might enjoy listening to these podcasts...
Tuning In To Scifi TV (covering all genre shows)
The Seeker Cast (Legend of the Seeker)