I know at least one person who moved to the US from China for graduate school and had no idea about Tiananmen. Mind you, it is not something I bring up, she just happened to bring it up during a conversation

It is pretty easy to find Chinese people talking about how few people (specially younger people) know about it too:

http://www.quora.com/China/What-percentage-of-Chinese-know-a...

http://observers.france24.com/content/20080605-tiananmen-squ...

Even those who have heard about it though, many probably haven't seen the pictures to truly understand the scale of it.

It was a crazy thought for me, too, but something to also consider is that there's some selection bias in the Chinese that you might talk to (i.e. Chinese people that talk to foreigners regularly) vs. ones that don't; the ones that do probably tend to know more, whereas the ones that don't probably fall into the "never heard of Tiananmen" category.

Maybe they did. Does that explanation mute the concern?