Rather than an email "recall" or "delete" command, I think an "amend" function could be useful. For instance, if an email was properly signed with the right key, it could amend/update a previously sent (similarly signed) email, while maintaining a revision history.

Need to make a small change (adding a forgotten attachment, for instance): no problem. Need to make a big change (e.g., removing your 2 am drunken rant), and manage to send it before the recipient has read the original? Better hope the recipient doesn't click on the "previous versions" button.

Within a few more years, people will have 1TB in their pocket, whether on an ipod-like device or flash drives on their keychains. With normal compression, that's about 100,000 songs.

If Governments(/business interests) were to succeed in making stuff unswappable online, sneakernet will return.