@omegadraconis
You got the idea, unix-like pipe would be a suitable connection .
As to prevent cheating and double spending, I am agree with
you it is hard to prevent in off-line transactions. However, it might be a habit for future bitcoin user to check immediately validity
of an offline transaction with on-line checker. Or they might only accept offline payment from a trusted party only.
It is problem with people and habit when using bitcoin, IMHO.
I gave it a little more thought here and wandered back to the time I used to work retail and use credit card machines. Most of them worked by having the card swiped and preforming an initial hold on the account for the amount. The amount was not actually finalized until the "batch" of transactions was run at the end of the day. I was thinking an system like this could work well for pseudo-offline system. You would still need to do an initial validation of some kind, like a bitcoin credit check or if the system for bitcoin reputation comes to be in effect, and then put the transaction into the batch for later processing.