Originally posted by reydp
But to my surprise, the same error was generated.
That's not surprising, that's an expected behavior. When you add a column with the dafault value to the table in one step, oracle actualy does it in three steps in the background:

Step 1: ALTER TABLE t ADD COLUMN c DEFAULT NULL;
Step 2: UPDATE t SET c = :default_value;
Step 3: COMMIT;

Now, step1 by itself is a transaction that has it's own implicit commit (sort of). So when you set the transaction to use a specific RBS before your ALTER TABLE ADD COLUMN command, that specific roollback segment was used only for step1. For step2 oracle immediately chooses another rollback segment. So in fact your effort to force the usage of a particular RBS has exactly the opposite effect - you was kind of preventing it from using the RBS of your choice for the rollback-intensive part of the operation.