The sci-fi technology that will truly enable mankind to efficiently explore space isn't fusion rockets, or space elevators, or any other grandiose machinery- its cryogenic sleep, or hibernation, or otherwise the ability to just conk out for long periods of time and wake up at your destination. Getting places in space is relatively easy, if you don't mind taking next-to-forever to get there. (In addition to the ITN, consider the bi-elliptic transfer: http://en.wikipedia.org/wiki/Bi-elliptic_transfer .)
"if you don't mind taking next-to-forever to get there."
If you want to be a (permanent-ish) space colonist that isn't wasted time.
As a counter example its continually proposed as a training / colonization system on short hops. If you want to test your 12-year rated life support system for a run to Jupiter, rather than sitting in low earth orbit the whole 12 years (boring!), why not try a leisurely ITN flight to the moon, in fact why not leave the ship/station in moon orbit when you're done, as a moon orbiting station?
If something goes horribly wrong, keep some large delta-v chemically fueled beast of a lifeboat around, worst case scenario on your "12 year mission to the moon" you'll never be more than many hours (well, OK, days worst case) from a splashdown on the earth via a very non-ITN path. Also your giant station/ship could be resupplied from the earth, at a considerably higher than ITN cost. Given your station/ship can move orbits very cheaply, your "lifeboat" could be extremely high performance.
The ITN would be a great way to help a manned (anywhere) expedition by slowly sending enormous quantities of cargo and base equipment long ahead of time.
A group that plans for the future would probably already be planning care packages and emergency packages to likely destinations. If it takes people 20 years to get to eventually really quickly fly to mars, then its no big deal if emergency O2 and first aid kits or whatever launched today take perhaps 15 years in transit.
But we do mind - unless we develop a cure for aging, there's no point in sending anyone anywhere if they're going to travel 50 years or more. By then, we'll lose interest in any data or resources we wanted.
Considering the very large distances, I suppose it would require very little in the way of manual intervention or energy. But I wonder how safe it will be unless at least one or two crew members are left awake to attend to the rest in case of emergencies.
That also leaves the question of what method to use for hibernation. Many mammals already do, but we may require a drastic change in our physiology or dive into genetic engineering (without knowing long-term effects, it could be dangerous) or discover some substance to inject into our blood or replace it altogether so we may lower our metabolism.
