The USD is already a digital currency. The Fed keeps a ledger of how much USD every bank has and transactions between banks, which enables banks to go back and forth between hard cash and digital currency. Note the Fed does not know how much each account has but that's the banks job.
From a banks perspective there is no reason to change things. They can trust checks from random people as long as the Fed clears the transaction between them and Bank X at the end of the day there in the clear.
You could argue that the physical bitcoins simply represent digital ones but the same can be said for USD as they are numbered. Coins are not currently numbered but represent a small fraction of the overall money supply.
You don't ever need to ship physical bitcoins, though. You can just digitize and transfer them. Which is preferred can be decided on the fly, which is quite a different situation than USD.
Personally, I'm not very convinced that physical bitcoins will really be meaningful: 1) cost of production is probably large compared to transaction fees, 2) counterfeiting issues. But I could easily be wrong there.
