In server-side C# code:

using Infragistics.WebUI.UltraWebGrid;
...
if (ultraWebGrid1.DisplayLayout.SelectedCells.Count > 0)
{
    UltraGridCell selectedCell = ultraWebGrid1.DisplayLayout.SelectedCells[0];

    // In practice, you'd cast this to the appropriate data type for the corresponding column
    object cellValue = selectedCell.Value;
}

In client-side JavaScript code:

var grid = igtbl_getGridById("<%= ultraWebGrid1.ClientID %>");
if (grid.SelectedCells.length > 0)
{
    var selectedCell = grid.SelectedCells[0];
    var cellValue = selectedCell.getValue();
}

Hello,

In the CSOM docs you can see a snippet for the selected rows:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.3/CLR3.5/html/WebGrid_Object_CSOM.html

In the like manner you can iterate through the selected cells:

for(var cellId in grid.SelectedCells)
{
var cell=igtbl_getCellById(cellId);
alert(cell.getValue());
}

If you need the value from current cell (active cell) you can do it like
this:
grid.getActiveCell().getValue()