He did a lot of voice-over work after TPF ended, but lately, his voice hasn't been heard recently. Has he retired/semi-retired from show business?

Well, more than 7 years after you asked, I can tell you that in 2007 he wrote (w/ his wife) his autobiography, "Reuben on Wry", which is available on Amazon:

http://www.amazon.com/Reuben-Wry-Memoirs-Dave-Madden/dp/1419681958/ref=sr_1_1?ie=UTF8&s=books&qid=1230918686&sr=1-1

I bought a copy of this book. It's an easy and interesting read. One notable thing is he states that Danny exaggerated his claims of what he did w/ Dave in his own autobiography. Dave is mostly retired now, living in Florida and travelling w/ his wife a good part of the time.

Last I saw him, he was interviewed for Danny Bonaduce's A&E Biography episode (as was Shirley Jones).