You just need access to subscriber data and a friends network. You could get enough data from volunteers, i.e. no recommendations until you give us the name of your five favorite podcasts.
I tried an informal experiment with this among my facebook friends. I asked them what podcasts they love. Then I put together an aggregator for those podcasts. It's been a great resource for me, but I don't think any of my friends use it.
I guess the answer is this -- if it's going to happen users have to want it and so far they haven't.