Using 1960s engineering (and many, many nuclear bombs), an Orion-powered starship could make it there in less than 150 years, one-way. And no slowing down; it would be a flyby.
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...
So yes, you could turn it around. But surprisingly it seems you can wait until a month before arrival to do so. The downside, however, is that your max velocity gets cut in half if you bring enough bombs to slow down.
With better explosives, you could get a higher max velocity and therefore closer to the "accelerate halfway, decelerate halfway" ideal case, but at the moment such explosives don't exist.
Do 36 days of acceleration at the beginning (or spread it over a year to reduce peak acceleration) -- go from 0 to 0.1c or whatever the cruise speed is.
Coast for ~100 years.
Spend the last year going from 0.1c to 0.
It does mean you have to carry twice as many bombs, and carry half of the bombs for 149 years, but bombs are really lightweight relative to the ship. It also means you can't just jettison the pusher plate, which is probably a bigger deal.
Orion is a great concept for tooling around the solar system easily, but what it buys us over interstellar distances is that it makes macroscopic payloads and reasonable travel times possible; they're still not easy.
[1] Table 2 in http://galileo.phys.virginia.edu/classes/109.jvn.spring00/nu...
I'd love to live in a world in 2030 or so where a private shuttle runs Earth-Mars every few weeks using Orion technology, since it wouldn't then need to operate on energy-conserving orbits.
Also, designing nukes that last for 150 years before being used is non-trivial (can't use tritium in the initiator, for example; it decays too rapidly for that).
It's not that slowing down is impossible...you just start throwing bombs the other direction. But by slowing down you increase your travel time, so the 150 year estimate is no longer valid.