"... we have argued that, as Wheeler’s picture of positrons as electrons moving backwards in time suggests, P-CTCs might also allow time travel in spacetimes without general-relativistic closed timelike curves. If nature somehow provides the nonlinear dynamics afforded by final-state projection, then it is possible for particles (and, in principle, people) to tunnel from the future to the past." 

The grandfather paradox (go back in time, kill your grandfather, prevent yourself from existing in the future) may not be a barrier after all...

A reader-friendly summary can be found here...

http://www.technologyreview.com/blog/arxiv/25494/

and the full paper is available here...

http://arxiv.org/PS_cache/arxiv/pdf/1007/1007.2615v2.pdf