I do not think it is possible to implement nested transactions that remain fully ACID compliant outside of the database engine itself as outside of your connection to the database may be many other connections you do not control and who knows what these other connections have done to the database.
in reply to Nested (sql) transactions
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James