Guys… shouldn't we all be in support of better I/O? Yes, USB is widespread. Yes, USB 3 is fast enough for external storage interconnects today. USB 3 is also far and away inferior in every technical respect. I, for one, very much hope that Thunderbolt gains widespread adoption, and a diverse peripherals ecosystem. I also hope the transition to photons is seamless, painless, and soon.
We would all benefit from having fewer types of ports on our computers. Faster, lower-latency, lower-level interconnects, and overall greater simplicity.
To answer the original question. It is quite possible to build an external version of anything you can hook into PCI Express using Thunderbolt. USB 3, eSATA, Firewire, 10Gbps Ethernet, DisplayPort, digital audio, an external GPU (in theory), custom solutions for specific vendors even.
Also, those of you comparing Firewire vs USB to Thunderbolt vs USB don't have your heads on straight. Apple made that mistake once, they aren't about to make it again. If Intel puts Thunderbolt on their Ivy Bridge chipset, and not USB 3… they can drive the market. Will they do that? Probably not, we don't know what the future holds. We don't know what Intel's long-term strategy is.
What we do know is this: Thunderbolt is better than USB 3 today. It is an open and diverse standard with a bright future (pun intended) that anybody can adopt today. It has the marketing and market force of today's Apple and Intel behind it (these are not niche players, if anybody has been keeping score). Apple's adoption is assured, we know that based on today's movement.
We will know more after March 2nd, iPad with a Thunderbolt port would be huge. I am pitching my lot in with Thunderbolt. It will be the interconnect of the future, and USB will not last. But call me crazy. After all, last time Apple supported a new universal Intel I/O, it never really caught on. I mean nobody uses USB these days.