In my opinion, the cloud presents the most interesting option. The idea of shunting all your backup data to a third party isn't new; organizations have been vaulting their backup tapes with third parties for decades. Doing it constantly and automatically over the Internet is an enticing concept.
Yet it doesn't take a rocket scientist to realize that the Internet will have to grow radically if everyone decides to replace tape with cloud services. To transfer a single LTO-4 cartridge's worth of data across a network circuit within a 24-hour window would require more than 75Mbps of dedicated bandwidth -- more than most of us have at our disposal. Can the cloud accommodate backups of our most mission-critical data? Sure -- but that's about it. Plus, if it's worth backing up, it's probably also worth archiving securely.
How can you replace tape? Really, you can't -- yet. However long tape's history may be, it's not over. If tape capacities keep pace with hard disk capacities and the growth of our data sets, tape will be around for a long, long time.