OK, so considering a situation like this: someone sends Bitcoins and doesn't include a transfer fee. What happens to those Bitcoins if the network "decides" not to deliver them to the destination address? Are they returned to the sender?
Good question. Anybody knows an answer?
They were never sent in the first place then and are still on the old address.
(might only be that some clients don't show them, because they are marked as "reserved to be sent")
Bitcoins are never "lost in cyberspace" they are always on a Bitcoin address.