There are several independently produced radio programs that are
carried on both commercial and public radio stations. These include
the Putumayo Radio Hour, Acoustic Cafe, American Routes and etown.
What is the business model for this type of programming: How are they
distributed? how do they cover their production costs? What are the
revenue streams both on the public and commercial side?

Shows such as American Routes are part of a distribution network such
as PRI(public radio international) or NPR.  There production budget
most likly comes from the station from which they tape their shows. 
That station makes their money from public contributions, and
purchased underwriting.

This may help: 
http://prx.org/;jsessionid=A1C971A149B69F01A54256F49BA46EEC.jvm1

This website is PRX, the Public Radio Exchange. It is "a web-based
marketplace for public radio pieces. Programmers find and air work
from other stations, independent producers and international
broadcasters. Producers - station-based or independent - license their
work directly to stations."

Programs such as Acoustic Cafe do have sponsors - edenfoods.com , for
example, is listed on their website as a sponsor. And they are
distributed through Segnet, a digital communications provider based
out of New Hampshire. http://www.segnet.com/services/index.html# .
Acoustic Cafe is also syndicated. Here is a website detailing what
that means, and the process you need to go through:
http://radio.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=radio&zu=http%3A%2F%2Fsyndication.net%2Fsyndicate.php

Oh, ok, well. Here we go: an about.com entry that has pretty much
everything you wanted to know, and some of the stuff I had already
found:
http://radio.about.com/od/syndicatingradioshows/ 

Hope that helps, and if you are going to try to start a new radio
show, best of luck.

Thanks ren8. That is very helpful information. One part of the
question remains unanswered: What is the business model? i.e. what are
the costs and revenues for an independently produced and distributed
radio program such as the ones mentioned in the original question?

Paulie3 --

There's also Democracy Now, which gets wide distribution through
college radio stations.  They're pretty straightforward about funding
sources, though this isn't exactly a "business model" --
http://www.democracynow.org/about.shtml

Best regards,

Omnivorous-GA