Question about detecting database changes

Hi, I have a question about getting some information about database changes.

Let's suppose I have a particular transaction ID that I happen to know has
been rolled back.

Is it possible to ask the database if there have been any committed
transactions subsequent
to that rolled-back transaction? Basically I'm would like to know if I will
see the same state
of data that I could see at the beginning of that transaction in the
transaction I am in right
now. I am using Postgres 9.2, but if it's only possible in a later version
I would be interested
in knowing that too.

Thanks very much!

--
--Dave Peticolas