Telerik blogs
NOTE: From the very beginning I would like to state that this custom solution is temporary and will most probably change with the next release of Silverlight Plug-In.
There is no straightforward approach to changing the Silverlight Download logo.

However, a workaround exists.

1) Download the modified TelerikSilverlight.js and add it to your project

2) Within the Default.aspx of your project, reference TelerikSilverlight.js 

<script type="text/javascript" src="TelerikSilverlight.js"></script>

3) Create a javascript function right bellow the previous line of code

<script type="text/javascript">
    function GetInstallHtml ()
        var imgUrl = "";
        var myInstallHtml = 'This is my custom text<br/>' + 
                                        '<div style="width: 205px; height: 67px; background-color: #FFFFFF">' + 
                                            '<img onclick="javascript:Silverlight.followFWLink({0});"' + 
                                            'style="border:0; cursor:pointer" src="' + imgUrl + '"' + 
                                            'alt="Get Microsoft Silverlight"/><br/>' +
                                        'This is my custom <a href="javascript:Silverlight.followFWLink({0});">link</a>.' +
        return myInstallHtml;

Sample Default.aspx


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