In terms of civilian deaths, its probably true that, had the US invaded the main islands of Japan, more civilians would have died than did in Hiroshima and Nagasaki. Its also true, as people have pointed out, that the firebombings of Dresden and other cities had the same effect as atomic bombs, just less efficient. America would doubtless have firebombed during the invasion, perhaps rendering the issue moot. Without targeting industrial/civilian locations at all during WW2, the war would have been very difficult. While perhaps noble, in a major war where the actors are concerned with survival, rules inevitably break down. Over asymmetrically strong participants "play by the rules" in war (see guerilla war, the American Revolution, etc.)
One of the "alternate history" questions I'd wonder on this subject: What would the Soviets have done? As you may recall, the Russians declared war on Japan at the very end, ousting them from North Korea and establishing Kim Il-Sung as communist dictator (Kim Jong-Il's father). I'd say its pretty clear Korea would've been better off had they not done that. Imagine if the Pacific war dragged on: instead of a divided Korea, we might have a fully gross Democratic People's Republic of Korea in Seoul, and a North/South split in Japan. That'd deprive millions of Koreans and Japanese of the democracy and prosperity they know today. Regardless of almost anything in the interrim, I'd say that's very important and good.
Still, on this infamous day, its an worthwhile question to ponder over. While the Japanese and German gov'ts were wrong for beginning that war, each death on both sides was tragic. Its not like shoemakers in Hiroshima or Berlin had any say in the matter.
