One feature in SL2 that was added between Beta 2 and RTW is a new constructor to the PropertyPath class that can be used from code. You can now do the following from code:

Storyboard.SetTargetProperty(myAnimation, new PropertyPath(MyCustomClass.MyCustomDP));

This is especially important if you are trying to target an animation at a custom attached property.  For example:

<DoubleAnimation Storyboard.TargetProperty="(custom:MyCustomClass.MyCustomAttachedDP)" .../>

Specifying these types of property path from a string representation presents a number of complications on the SL, and is not currently supported.  This piece of XAML currently raises an InvalidOperationException with message "Cannot resolve TargetProperty (custom:MyCustomClass.MyCustomAttachedDP)".

Please note that this only applies to custom attached properties - attached properties of built-in objects (e.g., Canvas.Top) as well as custom non-attached properties should work just fine.  As reference, an overview of the PropertyPath syntax can be found here.

Hopefully this will unblock you if you run into this limitation!
Ed