Here is the code to get a boolean item out of the list:

private static bool GetItemColumnAsBoolean(SPItemEventDataCollection item, string columnName)
{
    bool value = false;

    if (null != item[columnName])
    {
        string booleanString = item[columnName].ToString();

        // values are different in datasheet view
        if (null != booleanString)
        {
            booleanString = booleanString.ToLower().Trim();
            switch (booleanString)
            {
                case "-1": // user changed a boolean field to true in datasheet
                case "1": // field was already true in the item after a datasheet edit
                case "true": // normal edit (not using datasheet)
                    value = true;
                    break;
            }
        }
    }

    return value;
}

When an item was edited using the datasheet view you will be returned integer values:

• 0 for False
• -1 if the user clicks the checkbox in datasheet view and saves
• 1 if there was a boolean value anywhere in the item (and its value was already true)

Normally, you are just returned “True” or “False”.