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.

Any suggestions?

I'm not that familiar with Garageband, so I don't know if it's mixer is too oversimplified for this. But this is something you should be able to do with Jack. One option is to use the Jack AU plugin as an insert in a track. Then you can essentially treat Pd as an effects plugin. I haven't done this for a couple of years, though, and I did find under certain conditions it was buggy using Logic. What I usually ended up doing was sending the track to a different output channel other than the main outs. In other words, if the master track was channels 1-2, I would route the track that should go to Pd to be channel 3 or more. Those would be routed via Jack to Pd, and then Pd's outputs would be routed via Jack back to the DAW into separate tracks.

I haven't really used Garageband much, and I haven't used DAWs for quite some time, so sorry if that seems too general. From what I remember about Garageband, though, it has a very limited mixer environment, so you might not be able to get the routing you're look for, and it might not be a particularly good DAW to use for something like this. If that's the case, you might want to look at Ardour. I haven't gotten around to using it, but it's free and supposedly pretty damn good. Don't know if it has much in the way of instruments, though.

Also, if you want to get a better handle of polyphony in Pd (which is what I assume you mean by "multiple simultaneous midi notes"), I made a post in the Tutorials section here about using the [poly] object for that.