I recently read Michio Kaku's "Physics of the Impossible" - quite a good book, I highly recommend it. One of the sections was on time travel, and he repeated Stephen Hawking's observation that if time travel were ever to be possible, then why aren't we awash in time-traveling tourists now?
This is sort of taken for granted in this book, as elsewhere. And yet I cannot help but think that there are some assumptions there that ought not go unchallenged. It seems to me that there's a limitation on how long anything in the present day is going to be interesting to people in the future. There's really very little going on - it all seems important to us now, but in fifty years people will probably be mostly interested in a relative handful of things. In a hundred years, it seems to me that maybe only a few dedicated historians would come here, maybe to watch Obama's inauguration in person.
But it seems more likely to me that it'll take several hundred years, or thousands of years, to develop a working time machine. Who knows how long it would take to make the technology available to more than a lucky few. It took fifty years for low-orbital space travel to become available to a half dozen wealthy tourists, who had to train hard on the ground, then keep out of the way during a short mission. Who are their equivalents, I wonder, who are also more keenly interested in events of 1000 years ago than 2000 years ago, or 500? Why come back to the early 21st Century when there were such interesting things happening in Rennaisance Italy or in Roman-occupied Judea? (After all, a time machine, a camcorder, and an Aramaic-to-Chinese dictionary could clear up more than a few obnoxious arguments)
Let's face it, we're boring, and that's a strike against us. Speaking from a tourism point of view, we're the temporal equivalent of Ohio. Sure, we've got Obama's inauguration and the Iranian election, but in the thousand-year view of things they don't even amount to that huge ball of yarn.
On top of that, we're watching for time travelers. Or at least, we've managed to build ourselves a neat little surveillance society with lots of cell phone cameras, the concept of time traveling tourists, and "healthy" doses of novelty-seeking and paranoia.
Finally, it takes a special kind of mind to see today's successful experiments with visible-light invisibility and other negative-optics shenanigans, hypothesize a future civilization capable of time travel as a leisure activity... and still think that we'd actually be able to tell when the handful of bored tourists actually does show up.