I remember searching for reproductive systems during high school on Google Images to find some pictures for a science project. I'll leave it up to your imagination what I found.
Well. Google is now making it easier for other children to make the same mistake I did. Page Traffic Buzz has found that Google no longer blocks searches for "sex" or "nudity" under strict safe search guidelines. I have confirmed this finding myself. Under strict safe search, the images that come up under both "sex" and "nudity" are pretty innocuous as they only feature PG-13 images of couples in bed together or women covering their breasts. There are also a lot of images from Sex and the City.
The term "porn" still brings up the message that it can't find anything in regards to that search query. I took this opportunity to look up some other words and see if strict safe search would return any results. Searches for "S&M and "BDSM" returned no results claiming that safe search has found the content therein to be offensive to young eyes or delicate minds.
Searches for the proper scientific terms used for reproductive organs returned proper scientific results and images. Using slang like "cock" and "clit," however, returned no results as expected.
The strangest results, however, come from slang terms for deviant sexual acts. While the results don't return anything graphic, you can use context from the images that do show up to gather what they mean.
I'm sure I could do this all day, but I have other work to do. I'll leave the task of finding what Google does and does not block up to people who have more free time.
All of this doesn't give kids an excuse to go around laughing maniacally searching for "dirty words" on Google Image search. The words "sex" and "nudity" can offer more than just the lowest common denominator form of humor. Use this new freedom with responsibility or Google might revoke your right to.