Ok here is the situation. And apologies to Dzug, he posts some good stuff, I just get a bit tired at the amount of misinfo re: US visas.
There is an apparent contradiction re: US visas. You can enter on the Visa Waiver Scheme if you have not been arrested/charged with a crime of moral turpitude. On the other hand the embassy will tell you you have to apply for a visa, if you have any arrest/charge at all. Let's be honest, if the US applied the latter rule strictly, their tourism industry would collapse.
I have a Visa Waiver form in front of me. The box in question reads...
'B. Have you ever been arrested or convicted for an offence or crime involving moral turpiture or a violation related to a controlled substance; or been arrested or convicted for two or more offences for which the aggregate sentance to confinement was five years or more; or been a controlled substance trafficker; or are you seeking entry to engage in criminal or immoral activities?'
Summarising legalese is prone to confuse, but the way I read it moral turpitude involves some intent to harm others or society, not just yourself.
Here is the Embassy manual, read this and reach your own conclusion.
http://foia.state.gov/masterdocs/09fam/0940021 aN.pdf
...And having gone though the process, for heavens sake don't apply for a US visa unless you really have to!