I reworked the script and now it works:

// rasterize selected items

main();

function main()
{
// get all selected items
var items = document.getItems({
selected: true
});

if( !items.length ) {
Dialog.alert( "Please select art item(s) to be rasterized" );
return;
}

// create a new layer to hold rasterized items
var raster_layer = new Layer();
raster_layer.name = "raster";
raster_layer.activate();

// copy all selected items to new layer
// (traversed on reverse to preserve item order)
for( var i = items.length - 1; i >= 0; i-- ) {
items[i].copyTo( raster_layer );
}

// group and rasterize
var group = Group( raster_layer.children );
var raster = group.rasterize( 'rgb', 72, 0 );

// remove group of selected items so the raster layer only contains the image
group.remove();
}

Not really sure why the static function Item.rasterize didn't work in my first attempt.

Aloha - Beau.