Mountable Apps

Hi   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

Gary