Here is a great podcast about extreme metal I found some time ago while looking for metal podcasts on iTunes. Jason and Mark are very knowledgable of the underground metal scene during the 80s and into present times both having worked on fanzines and even having friends that work for Decibel Magazine.
And they actually talk about real extreme metal. Sometimes they dip into metalcore but its usually the more dissonent types like Converge or The Dillinger Escape Plan. They usually pick an album, or band and the shows format depends on that.
If its a band they play a few songs from the band's eras or important albums/eps and discuss what was going on at the time and give a perspective from their point of view. They also like to throw some fun and interesting facts out there. If its an album they basically do the same thing but with a focus, obviously, on that album. They also do a Top (insert number here) releases of the year show at the end of every year that is fun to listen to.
They have gotten me interested in a lot of bands who to me were new, Anata, Katatonia, Gates of Ishtar, Amorphis, and Edge of Sanity. Definitely check them out if you want to hear some good radio show like stuff about extreme metal bands. Much props to the guys for doing a Bolt Thrower show!
Check out the guys at the Requiem Metal Podcast site where you can download their shows, or find them on iTunes.