A couple of years ago I had the delightful experience to chat with one of the foremost scientists in the field, Dr Fay Dowker, and I posed this exact question to her: Is time travel possible, and if so, can we go back and fiddle with the past?
Her answer was, in pure physicist style, somewhat long-winded and most of it went right over my head. But, essentially, her argument that was time is a one-way trip. We can travel into the future by approaching the speed of light (re E = MC2 and all the rest) but can't go back.
That said, she was humble and open enough to the possibility that we know very little about how time actually works so maybe there's some way to go back in time, but at present we don't think so.
More about Fay Dowker here:
Fay Dowker - Wikipedia, the free encyclopedia BBC The Next Big Thing - Anti-matter Fay Dowker: Information and Much More from Answers.com http://loops05.aei.mpg.de/index_file...les/dowker.pdf (this one's a PDF that may take a moment or two to load)