The above is not really giving the full story, and could lead to trouble. The various spent conviction legislation simply governs whether or not such information can be released, and in what circumstances - it does not remove the information. Immigration inquires will not reveal the information once spent, but it still exists.
What must always be considered, is that the spent convictions provisions only relate to disclosure of the source record from Australia. As this thread relates to US in particular, if they already have information stored in a US database, that will not be covered in any way by Australian spend convictions legislation.
Anybody who has in the past travelled to the US (especially with a Visa) at a time prior to the conviction becoming spent, would be most unwise to rely on an Australian interpretation of whether or not they need to reveal the information when applying to trael to the US - as it is the US interpretation which is going to apply to the situation.