getCellByColumnUniqueName

Gives you the cell specified by a row and a column unique name.

getCellByColumnUniqueName(gridRow, columnUniqueName)
gridRow	GridDataItem	The row which holds the cell.
columnUniqueName	String	The UniqueName of the column which identifies the cell uniquely.

In order to get the cell value, you need to use the innerHTML property:

function GetFirstItemContactNameCell() {
    var masterTable = $find("<%=RadGrid1.ClientID%>").get_masterTableView();
    var contactName = masterTable.getCellByColumnUniqueName(masterTable.get_dataItems()[0], "ContactName").innerHTML;
}

The ShowHeader attribute for the RadGrid/MasterTableView should be set to true in order to use this method.