Hi,
i've had the same problem, many thanks to Jens and Rajeshwar for the
solution!
I've extended Column and TextColumn:
public abstract class TooltipColumn<T, C> extends Column<T, C>{
	private final Cell<C> cell;
	public TooltipColumn(Cell<C> cell) {
		super(cell);
		this.cell = cell;
	}
	public abstract SafeHtml getTooltipValue(T object);
	@Override
	 public void render(Context context, T object, SafeHtmlBuilder sb) {
		SafeHtmlBuilder mysb = new SafeHtmlBuilder();
		cell.render(context, getValue(object), mysb);
		mysb.appendHtmlConstant("<span>");
		mysb.append(getTooltipValue(object));
		mysb.appendHtmlConstant("</span>");
		sb.append(mysb.toSafeHtml());
	  }
}
public abstract class TooltipTextColumn<T> extends TooltipColumn<T,
String> {
	  /**
	   * Construct a new TextColumn.
	   */
	  public TooltipTextColumn() {
	    super(new TextCell());
	  }
}
CSS:
	  .CssCellTableCell span {
	 		display:none
	 	}
	  .CssCellTableCell:hover span {
	 	border:1px solid #e6e3e5;
	 	DISPLAY: block;
	 	Z-INDEX: 1000;
	 	PADDING: 0px 10px 0px 10px;
	 	POSITION:absolute;
	 	float:left;
	 	background:#ffffd1;
	 	TEXT-DECORATION: none
	}
Add a Column:
		TooltipTextColumn<Todos> aTextColumn = new
TooltipTextColumn<Todos>() {
			@Override
			public String getValue(Todos object) {
				return object.getText();
			}
			@Override
			public SafeHtml getTooltipValue(Todos object) {
				return new
SafeHtmlBuilder().appendEscaped(object.getText()).toSafeHtml();
			}
		};
		cellTable.addColumn(aTextColumn, "Text");