Well, that didn't take long. The Library of Congress, if a work has not been deposited, can contact the publisher and request their two copies. If the two copies are not delivered to them within three months of the request, the publisher can be fined $250. An additional $2500 can be fined if the violation is found to be willful or flagrant.
( information from
http://www.eshieldkret.com/copyright/deposit.shtml )
Everything I can find, though, says that the mandatory deposit requirment does not affect copyright.
Since the deposit requirement is for the "best edition", the only way that I can think of that someone could get caught out by this would be if the "best edition" was made in such a way that it wouldn't be possible to recreate it and deliver it in three months, and you didn't keep two copies of that "best edition" that you could send in if needed.
Thus, for those publishing ebooks, even if the law were to be changed to require ebooks to be deposited, there's no problem, as long as you keep a copy so you can provide when/if asked.
Of course, you can also request "Special Relief". In the case where you can't provide the "best edition" any more, that would probably be what you'd need to do. If you couldn't provide any version any more (e.g., the ebook was no longer being offered anywhere and you'd deleted your own copies of it, so you'd have to literally rewrite it in order to provide a copy), I'd imagine the "Special Relief" would have to be requested as well.