Ahí van los botones redondos en xaml. Es una versión hecha en un ratito sin refinar, pero para que veais que se puede hacer cualquier cosa en poco tiempo, he redefinido el estilo del tipo Button y se lo aplico a un botón en el formulario.
Para verlo, copiad el xml y lo pegais en un archivo con la extensión .xaml
<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Page1"
>
<Page.Resources>
<Style x:Key="RoundButton" BasedOn="{x:Null}" TargetType="{x:Type Button}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Grid>
<Ellipse Stroke="#FF000000" x:Name="backGroundEllipse">
<Ellipse.Fill>
<LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#FFF0F0EA" Offset="0.909"/>
</LinearGradientBrush>
</Ellipse.Fill>
</Ellipse>
<Ellipse Opacity="0.305" RenderTransformOrigin="0.5,0.5" Fill="{x:Null}" Stroke="#FF0002FF" StrokeThickness="2" x:Name="focusEllipse">
<Ellipse.RenderTransform>
<TransformGroup>
<ScaleTransform ScaleX="1" ScaleY="1"/>
<SkewTransform AngleX="0" AngleY="0"/>
<RotateTransform Angle="0"/>
<TranslateTransform X="0" Y="0"/>
</TransformGroup>
</Ellipse.RenderTransform>
</Ellipse>
<ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" RecognizesAccessKey="True"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsFocused" Value="True"/>
<Trigger Property="IsDefaulted" Value="True"/>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderThickness" Value="1,1,1,1"/>
<Setter Property="Stroke" TargetName="backGroundEllipse" Value="#FF000000"/>
<Setter Property="Stroke" TargetName="focusEllipse" Value="#FFFFB600"/>
<Setter Property="RenderTransform" TargetName="focusEllipse">
<Setter.Value>
<TransformGroup>
<ScaleTransform ScaleX="0.98" ScaleY="0.98"/>
<SkewTransform AngleX="0" AngleY="0"/>
<RotateTransform Angle="0"/>
<TranslateTransform X="0" Y="0"/>
</TransformGroup>
</Setter.Value>
</Setter>
<Setter Property="Opacity" TargetName="focusEllipse" Value="0.57"/>
<Setter Property="StrokeThickness" TargetName="focusEllipse" Value="2"/>
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter Property="Fill" TargetName="backGroundEllipse" Value="#FF9B9B9B"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False"/>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsKeyboardFocused" Value="True"/>
<Condition Property="IsMouseOver" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Stroke" TargetName="backGroundEllipse" Value="#FF000000"/>
<Setter Property="StrokeThickness" TargetName="backGroundEllipse" Value="1"/>
<Setter Property="Stroke" TargetName="focusEllipse" Value="#FF000175"/>
<Setter Property="StrokeThickness" TargetName="focusEllipse" Value="3"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Page.Resources>
<Grid>
<Button Style="{DynamicResource RoundButton}" Height="69" HorizontalAlignment="Left" Margin="8,8,0,0" Name="button1" VerticalAlignment="Top" Width="75">Button</Button>
</Grid>
</Page>
Juan M. Servera