To view the original details of a transaction downloaded from your bank, select the transaction and choose Transaction> Show Original Import Info. This is the case for transactions which you have edited and saved, and even works for merged transactions.

Note that merged transactions are created when you resolve duplicates manually. While iBank 5 does a great job at matching transactions and eliminating duplicates automatically, occasionally a duplicate transaction gets through. When this happens you can merge them by simply dragging and dropping one of the duplicate transactions over the other.