I would suggest setting cellType to dojox.grid.cells.Bool instead of formatting. Formatting gives you great freedom, but also the responsibility of collecting data from all the checkboxes (for all rows) afterwards, Something like this as a structure record should do the trick:
{
name: "is awesome?",
width: "auto",
styles: "text-align: center",
type: dojox.grid.cells.Bool, editable: true
}
, (, ItemFileWriteStore), , , :)