Hmm.
This works:
<Rectangle x:Name="MyAnimatedRectangle" Canvas.Left="100" Canvas.Top ="100" Width="100" Height="100" Fill="Blue">
<DoubleAnimation Storyboard.TargetName="MyAnimatedRectangle" Storyboard.TargetProperty="Width" From="200" To="400" Duration="0:0:3" RepeatBehavior="6"/>
... but this doesn't:
<DoubleAnimation Storyboard.TargetName="MyAnimatedRectangle" Storyboard.TargetProperty="Canvas.Left" From="200" To="400" Duration="0:0:3" RepeatBehavior="6"/>
Both Width and Canvas.Left are properties of Rectangle... so is this a bug? I'd assume so. (It's the same in firefox and explorer 7).
Later. Ok I found out why -- the property syntax needs to have brackets around complex statements for some weird reason, so it needs to be (Canvas.Left). That works fine.