npr podcasts are mp3 files, but have a content-type of application/itunes.  squeezecenter/slimserver doesn't recognize this problem and can't play them (although the audio data is playable).

here's a snippet of the server log

[08-03-04 15:46:54.2470] Slim::Player::Squeezebox::stream (1012) This player supports direct streaming for http://podcastdownload.npr.org/anon.npr-podcasts/podcast/13/
87873336/npr_87873336.mp3?dl=1 as http://podcastdownload.npr.org/anon.npr-podcasts/podcast/13/87873336/npr_87873336.mp3?dl=1, let's do it.
[08-03-04 15:46:54.2842] Slim::Player::Squeezebox::stream (1043) setting up direct stream (134608137:80) autostart: 3.
[08-03-04 15:46:54.2851] Slim::Player::Squeezebox::stream (1044) request string: GET /anon.npr-podcasts/podcast/13/87873336/npr_87873336.mp3?dl=1 HTTP/1.0
Cache-Control: no-cache
Connection: close
Accept: */*
Host: podcastdownload.npr.org
User-Agent: iTunes/4.7.1 (Linux; N; Debian; i686-linux; EN; iso-8859-1) SqueezeCenter/7.0/17491
Cookie: v1st=8B3B5702CCE769D0
Cookie2: $Version="1"
Icy-Metadata: 1

[08-03-04 15:46:54.2886] Slim::Networking::Select::select (245) Error: Select task failed: Can't call method "connectionID" on an undefined value at /home/dan/source/s
queezecenter-7.0-17491/Slim/Web/JSONRPC.pm line 403.
[08-03-04 15:46:54.3750] Slim::Player::Squeezebox2::directHeaders (284) Processing headers for direct streaming:
HTTP/1.0 200 OK
Server: Apache
ETag: "14907de2c2bdaa57656f98dcdc0c3ce0:1204598087"
Last-Modified: Tue, 04 Mar 2008 02:34:47 GMT
Content-Length: 22062317
Content-disposition: attachment; filename=npr_87873336.mp3
Date: Tue, 04 Mar 2008 23:46:54 GMT
Connection: close
Accept-Ranges: none
Content-Type: application/iTunes

[08-03-04 15:46:54.3764] Slim::Player::Squeezebox2::directHeaders (343) Processing 9 headers
[08-03-04 15:46:54.3774] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Server: Apache
[08-03-04 15:46:54.3783] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: ETag: "14907de2c2bdaa57656f98dcdc0c3ce0:1204598087"
[08-03-04 15:46:54.3793] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Last-Modified: Tue, 04 Mar 2008 02:34:47 GMT
[08-03-04 15:46:54.3802] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Content-Length: 22062317
[08-03-04 15:46:54.3811] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Content-disposition: attachment; filename=npr_87873336.mp3
[08-03-04 15:46:54.3820] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Date: Tue, 04 Mar 2008 23:46:54 GMT
[08-03-04 15:46:54.3829] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Connection: close
[08-03-04 15:46:54.3838] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Accept-Ranges: none
[08-03-04 15:46:54.3847] Slim::Player::Protocols::HTTP::parseDirectHeaders (267) header-ds: Content-Type: application/iTunes
[08-03-04 15:46:54.3885] Slim::Player::Squeezebox2::directHeaders (396) Got a stream type:  bitrate:  title: 
[08-03-04 15:46:54.3904] Slim::Player::Squeezebox2::directHeaders (472) Direct stream failed for url: [http://podcastdownload.npr.org/anon.npr-podcasts/podcast/13/8787
3336/npr_87873336.mp3?dl=1]
[08-03-04 15:46:54.3914] Slim::Player::Squeezebox2::failedDirectStream (600) Oh, well failed to do a direct stream for: http://podcastdownload.npr.org/anon.npr-podcast
s/podcast/13/87873336/npr_87873336.mp3?dl=1 []
[08-03-04 15:46:54.3925] Slim::Player::Source::errorOpening (1609) Error: While opening current track, so mark it as already played!
[08-03-04 15:46:54.3939] Slim::Player::Source::errorOpening (1609) Backtrace:

   frame 0: Slim::Utils::Log::logBacktrace (/home/dan/source/squeezecenter-7.0-17491/Slim/Player/Source.pm line 1609)
   frame 1: Slim::Player::Source::errorOpening (/home/dan/source/squeezecenter-7.0-17491/Slim/Player/Squeezebox2.pm line 605)
   frame 2: Slim::Player::Squeezebox2::failedDirectStream (/home/dan/source/squeezecenter-7.0-17491/Slim/Player/Squeezebox2.pm line 474)
   frame 3: Slim::Player::Squeezebox2::directHeaders (/home/dan/source/squeezecenter-7.0-17491/Slim/Networking/Slimproto.pm line 520)
   frame 4: Slim::Networking::Slimproto::_http_response_handler (/home/dan/source/squeezecenter-7.0-17491/Slim/Networking/Slimproto.pm line 381)
   frame 5: Slim::Networking::Slimproto::client_readable (/home/dan/source/squeezecenter-7.0-17491/Slim/Networking/Select.pm line 243)
   frame 6: (eval) (/home/dan/source/squeezecenter-7.0-17491/Slim/Networking/Select.pm line 243)
   frame 7: Slim::Networking::Select::select (./slimserver.pl line 503)
   frame 8: main::idle (./slimserver.pl line 453)
   frame 9: main::main (./slimserver.pl line 1005)