Pointing to a paper recently accepted for publication, Paul Gilster explains how the best ideas for fast space travel must cope with another, non-technological, constraint.
For example, while "sundiving" missions designed to place a solar sail-powered craft near the sun for maximum outward acceleration - one study suggests that deployment at .1 AU could send such a craft to 200 AU in only 2.5 years! - might be accomplished, designers would have to account for General Relativistic effects in their craft's navigation or find it potentially millions of kilometers off course by the time it reaches the Oort Cloud. As one commenter points out, MESSENGER's flight calculations have also taken General Relativity into account. And another points to this Technology Review blog entry on "relativistic navigation."