Probably the best description I've seen of what people think the press conference is going to be about came from my friend Sean Carroll at Caltech: it seems that an experimental group observing the Cosmic Microwave Background is going to announce that they have seen direct evidence of perturbations of that background caused by gravitational waves in the first instants of the Big Bang. (As Sean explains, today our direct experimental data on the early universe extends back to about one second after the start of the Big Bang. This observation would push that back to an astounding 10-35 seconds after the start.)
I won't try to explain the physics here, since it's really not my specialty. The intriguing thing is that, as far as I can tell, most people were not expecting to see any actual detection of this signal from the current generation of experiments: other data suggested that the current experiments would only be able to set "less than this threshold" sorts of limits. So this impending announcement would seem to imply one of four things: 1) The signal is much stronger than expected, which would be Very Exciting(TM) for physics, 2) The experiment turned out to be more sensitive than expected, which would presumably involve either really good luck or some neat improvements in data analysis algorithms, 3) The announcement is merely of strongly suggestive evidence rather than a true discovery-level result, which would make the "major discovery" press conference seem quite overblown, or 4) Someone messed up their analysis and/or got fooled by a statistical fluctuation, which after all this hoopla would probably wind up ending multiple careers. (I can guarantee that the experimental team here is painfully aware of all these possibilities. But then, so were the folks who claimed to have seen neutrinos moving faster than light a few years back.)
So yeah. It sounds like the actual science talk will begin at 10:45 (with papers and data going online at the same time). So watch the news, or at least the blogs! It should be exciting.