the KVR DSP forum: http://www.kvraudio.com/forum/viewforum.php?f=33
and the music-dsp mailing list: http://music.columbia.edu/mailman/listinfo/music-dsp
Even armed with all the resources in the world, though, I cannot stress how important just jumping in is. Write a couple of basic plugins and you'll get a really good feel for how the pieces fit together, what's important in the algorithms, etc.