If you have a need to use a checkbox TreeGrid that works similarly as the TreePanel, and can't wait for the GXT v3.0, you may want to consider the attached custom TreeGrid. I am not aware of any issues with it at this time. You would use the CheckboxTreeGrid as you normally would with a TreeGrid, with the following difference configuration for the first column that contains the check boxes.
Code:
ColumnConfig name = new ColumnConfig("name", "Name", 300);
name.setRenderer(new CheckboxTreeGridCellRenderer<ModelData>(true));
Thanks,
Phil