Assault only constitutes 'moral turpitude' if it's:
(a) with intent to kill ; or
(b) with intent to commit rape ; or
(c) with intent to commit robbery ;
(d) with intent to commit serious bodily harm ; or
(e) with a dangerous or deadly weapon.
Other forms of assault do not constitute moral turpitude:
http://www.state.gov/documents/organization/86942.pdf
However the US Department of Homeland Security's rules state that anyone who has ever been arrested (even if totally innocent) or convicted of any offence is ineligible to travel under the Visa Waiver Program (i.e. using an 'ESTA'):
http://london.usembassy.gov/vwp3.html
(i.e. you must seek a visa).
You should schedule an interview as soon as possible:
http://london.usembassy.gov/vwp3.html
while seeking an ACRO Police Certificate:
http://london.usembassy.gov/add_crime.html
While the US authorities have no direct access to UK criminal records, a great deal of information is passed on under the guise of 'anti-terrorism' (even, no doubt, when the individuals involved are in no way suspected of terrorist activities). It would be extremely unwise to try to cheat the system.
Chris