Mountable Apps

I am very interested in mountable applications in rails 3. I would
like to know if this is an ‘engines’ replacement as I have a number of
engines plugins and I need to know the way forward for converting them
either to ‘rails 3 engines plugins’ (when engines becomes rails 3
compliant) or simply ‘mountable apps / plugins’.

Can anyone give me some direction on this one please ?

Many Thanks