Several reports have come in about the the mythical drag n’ drop multi-column grid plugin.

Gridster

This is it, the mythical drag-and-drop multi-column grid has arrived. Gridster is a jQuery plugin that allows building intuitive draggable layouts from elements spanning multiple columns. You can even dynamically add and remove elements from the grid. It is on par with sliced bread, or possibly better. MIT licensed. Suitable for children of all ages. Made by Ducksboard.

Ha, I appreciate the "mythical" reference :)

Open Assembly

Open Assembly was developed by Pressed Web, featuring a highly customizable interface than employs a draggable version of Masonry.

Awesome to see these solutions arise. For the time being, my branches of Masonry and Isotope will continue to be un-draggable. But both these options demonstrate that it can be done.