This is what I have been using:

   UIElement element = this.DisplayLayout.UIElement.ElementFromPoint(e.Location);

  UltraGridCell cell = element.GetContext(typeof(UltraGridCell)) as UltraGridCell;

      cell.Value = !((bool) (cell.Value));

Here's some sample code I whipped up that works pretty well. Note that the column's CellActivation has to be se to ActivateOnly (or maybe NoEdit) in order for this to work properly. 

        private void ultraGrid1_MouseUp(object sender, MouseEventArgs e)

        {

            // Get the grid.

            UltraGrid grid = (UltraGrid)sender;

            // Get the last UIElement entered.

            UIElement lastElementEntered = grid.DisplayLayout.UIElement.LastElementEntered;

            // If there is no element, return.

            if (lastElementEntered == null)

                return;

            // Get the cell that the element is in.

            UltraGridCell cell = lastElementEntered.GetContext(typeof(UltraGridCell)) as UltraGridCell;

            // If there is no cell, return.

            if (cell == null)

                return;

            // Get the cell's editor as a CheckEditor.

            CheckEditor checkEditor = cell.EditorResolved as CheckEditor;

            // If it's not a CheckEditor, we are not in a CheckBox cell, so return.

            if (checkEditor == null)

                return;

            // Get the CheckIndicatorUIElement. This is the actual checbox within the cell.

            // See if the lastElementEntered is a CheckIndicatorUIElement.

            CheckIndicatorUIElement checkIndicatorUIElement = lastElementEntered as CheckIndicatorUIElement;

            // If the lastElementEntered is not a CheckIndicatorUIElement, so if one of it's ancestors is.

            if (checkIndicatorUIElement == null)

                checkIndicatorUIElement = lastElementEntered.GetAncestor(typeof(CheckIndicatorUIElement)) as CheckIndicatorUIElement;

            if (checkIndicatorUIElement != null)

            {

                // If checkIndicatorUIElement is not null, it means the click was on the CheckBox

                // and we want to toggle the cell value.

                bool cellValue = (bool)cell.Value;

                cellValue = !cellValue;

                cell.Value = cellValue;

            }

            else

            {

                // If checkIndicatorUIElement is null, it means the click was in a checkbox cell, but

                // not on the CheckBox itself, so select the row.

                cell.Row.Selected = true;

            }

        }

 Hi Hamanta,

    UltraGridColumn is not a UIElement, so I'm not sure what your code is doing, but it doesn't sound right to me. Try searching the Infragistics KB for articles on the ElementFromPoint method. There are articles with sample code that show how to get the cell that the mouse is over.