I have this in a custom table form (which uses Biz Forms). The syntax is for v7 but should work for v6 as well.
protected void customTableForm_OnAfterSave(object sender, EventArgs e)
{
CustomTableForm ctf = (CustomTableForm)sender;
// need to programatically get the primary key column
IDataClass idc = DataClassFactory.NewDataClass("your.class.name");
// get the id of the last inserted item
int lastInsertedId = ValidationHelper.GetInteger(ctf.BasicForm.Data[idc.IDColumn], -1);
}