First: I have a Mac, OSX 10.7 and an old-ish Casio CTK-720 as midi-in.
I have and love Pd, but I like the instrument options and recording interface in Garageband (plus the fact that it can handle multiple simultaneous midi notes in from my casio, something I haven't figured out how to do in Pd.) What I'd like to be able to do is route midi in from the casio to Garageband, send audio out from Garageband to Pd (for various effects Gband can't do/fun), and then route the audio back into Gband to record while being able to monitor it. I've messed around with both Jack and Soundflower, and I can't figure out how to make this dream come true.
I think I have two 'outs,' maybe, namely to use something else to either generate the synth sounds or to do the recording/monitoring. But I'm not sure what I'd use.