We just entered a support note I wanted to raise awareness of because there are so many SharePoint 2007 users out there. Our detailed write-up and solution is in our forum, but in a nutshell, if you use our IntelliGantt2007.WSP solution on an out-of-the-box SharePoint 2007 configuration on an out-of-the-box Windows Server 2003 configuration, then the IntelliGantt.xap Silverlight package will not be served by IIS.
The solution is to add .xap file extensions to the list of approved MIME Types for IIS.
Thankfully, SharePoint 2010 and Windows Server 2008 recognize .xap files by default so this is not an issue for newer infrastructure.
Nor is this an issue when using the SharePoint 2007 Rich Content Web Part to view .xap files. It's only with the 2007 specific WSP solution.