Just for the record Naoko, Ecchi is not equal to Hentai as you stated.
Perhaps... and perhaps not. I think depends on how you think of the terms.
As English loan words, "hentai" refers specifically to pornographic anime/manga/games (i.e. acts of sex can been seen). "Ecchi" refers to sexual images that don't depict sex (i.e. a topless girl).
But, for as they're used in Japanese (from my understanding, at least)...
"Ecchi" is the Japanese pronunciation of the English letter "H". H can refer to anime/manga/games of any level of sexual content (from images of nudity to images of sex). i.e. H CG, H-anime, etc.
"Hentai" just means pervert, and refers to a person who is perverted. Very rarely do Japanese use the term to refer to adult content.
The most commonly used term in Japan to refer to adult content is "ero" (short for erotic/erotica). i.e eroge (erotic game).