Can you try to create HTML page with following content and run it (I've just tried it myself, and it works for me - if it works for you too, then there is probably some problem with instantiation of SL plugin in ASP.NET):
<html>
<head>
<title>Silverlight Project Test Page</title>
<script id="slscript" type="text/xaml">
<Grid
xmlns="http://schemas.microsoft.com/client/2007"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="250" Height="500">
<Grid.Resources>
<Style x:Key="DefaultStyle" TargetType="TextBlock">
<Setter Property="FontSize" Value="40"/>
</Style>
</Grid.Resources>
<TextBlock Text="MyText" Style="{StaticResource DefaultStyle}" />
</Grid>
</script>
<script type="text/javascript">
function onSilverlightError(sender, args) {
alert("onSilverlightError: " + args.errormessage);
}
</script>
</head>
<body>
<div id="silverlightControlHost">
<object type="application/x-silverlight" width="600" height="600">
<param name="source" value="#slscript"/>
<param name="onerror" value="onSilverlightError" />
</object>
</div>
</body>
</html>
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