As you state, the forms only refer to 'arrests and convictions'. (So, as I previously posted and you've correctly indicated, you can lawfully answer 'None' since your caution was not preceded by an arrest).
However the actual wording of the US Immigration and Nationality Act states:
" . . . any alien convicted of, or WHO ADMITS HAVING COMMITTED, or who admits committing acts which constitute the essential elements of -
(I) a crime involving moral turpitude (other than a purely political offense) or an attempt or conspiracy to commit such a crime, or
(II) a violation of (or a conspiracy or attempt to violate) any law or regulation of a State, the United States, or a foreign country relating to a controlled substance (as defined in section 802 of title 21), is inadmissible"
http://www.fourmilab....t8-12-II-II-1182.html
Since a person can't be cautioned unless they admit their guilt, it could reasonably be argued that any who has been cautioned for an offence involving 'moral turpitude' or 'controlled substances' has admitted carrying out that offence (thus rendering them inadmissible for entry to the USA).
However your caution is clearly unrelated to 'controlled substances' and it definitely doesn't seem to be 'moral turpitude (
http://www.state.gov/...rganization/86942.pdf ), you're unaffected by any such concerns.
Chris