As for Don's observation regarding the redo log, yes, Oracle's design means that readers asking for a row since modified by another user "read around" the altered row to get the original.
I think you mean "rollback segment" instead of redo log in the paragraph I'm quoting. Oracle uses rollback segments for transactional behavior (the "A" and "I" in ACID), and redo logs to ensure that committed transactions do not get lost (the "D" in ACID).
