Here's another thread where I try to explain what is happening and why you shouldn't change the standard behavior: http://forum.blubrry.com/index.php/topic,1694.msg4307.html
These are web standards, you can manipulate them but doing so has dyer consequences. Lets toss out the fact that doing what you want would be a disservice to web accessibility; lets focus on the number 1 reason you would not want to force a "save-as" dialog for your users. Doing so will Limit who can download your media. Read on...
First, lets explain whats happening in a browser such as Windows Internet Explorer. When you click on a link with the content type "audio/mpeg", the light bulb goes off in Internet Explorer and says, "Hey I know this audio/mpeg, my buddy Windows Media Player can play it!". Internet Explorer then skips the whole ask to download and proceeds to save the download automatically to Windows Media Player for instant playback.
You can "fool" Internet explorer by changing the content type to "application/octet-stream", then it will not know what to do with the file and pop up a save-as dialog. If you do this, you then alienate other browsers such as the ones on Android/iPhone devices. iPhone/Android browsers for security purposes don't allow downloads of unknown types, thus limiting who can download your media severely.
Now we know for a fact iTunes desktop software, iPhone/iPad and Android devices will not allow downloads of unknown content types, and we know from Blubrry Statistics that 50-90% of a podcasts download comes from iTunes desktop alone, it would be a very <u>very</u> bad idea to change the content type.