Found this online, it proved VERY helpful and wanted to pass it along.
Silverlight Error #2104 Could not download the silverlight application in IIS6
If you receive this error while trying to deploy your Silverlight application, make sure you did the following things:
1. Adjust the Content Expiration Setting
You should adjust the content expiration setting to 1 minute. The following procedure outlines how to do this with IIS.
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Right-click your Web Site and select Properties from the context menu.
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Select the HTTP Headers tab and click "Enable Content Expiration".
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Set the content to expire after 1 minute.
2. Register MIME Types and File Extensions
You must register some MIME types and file extensions so that the browser on the client's system can load the correct handler. You need to add the following types:
Extension
MIME Type
.xap
application/octet-stream
.xaml
application/xaml+xml
The .xap needs to have that mime type, in order to be able to be downloaded.
3. Execute permissions must be Scripts Only.
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Right-click your Web Site and select Properties from the context menu.
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Select the Directory tab and change "Execute Permissions" to Script only.
Hope this helps.