Well, I just checked all through Wikipedia (a nice little wiki-walk), eventually settling on Comparison of American and British English, where white literally in the first sentence of the section on Numbers it is written:
When saying or writing out numbers, the British insert an and before the tens and units, as in one hundred and sixty-two or two thousand and three. In the United States it is considered correct to drop the and, as in one hundred sixty-two or two thousand three.
I believe that is is mainly because of the older American favoritism for saying the decimally written fractions as though they were longhand fractions. For instance, when saying the number 3854.12 an American is likely to say thirty-eight hundred fifty-four and twelve hundredths, rather than the (presumedly) more "British" method of three thousand, eight hundred and fifty-four point twelve. Another example of this is money, whereas I've heard that £4.20 would be four pounds twenty, $4.20 would be four dollars and twenty cents.