Gwern already linked to the same page previously. I've updated on the information, however, in my time at school I also did a research project on the atom bombing, and the sources I read for the project (which are not online, at least as far as I know,) cited Japanese military officials who were of the opinion that their country would have continued to resist, even to the point of a land invasion, and that the bombings were instrumental in changing that.

There are certainly good reasons to suspect that Japan might have surrendered soon under the same terms even without the dropping of the bombs, but it's also not as if there is a dearth of evidence suggesting that the bombings were a significant factor.