Many people in Toronto were wondering how Mayor Rob Ford was able to enter the US if he has a drunk driving charge in the US and a possible charge of marijuana that was dropped.
The Program Director of Pardons Canada was recently interviewed by CITY TV News in this recent article about this topic: http://www.citynews.ca/2014/05/06/rob-ford-never-officially-entered-united-states-report/
It turns out that Mayor Rob Ford was likely turned away at the US border recently because of his criminal past. There will be no NEXUS privileges for Rob Ford.
Indeed we receive hundreds of calls each week from Canadians who get denied entry to the US due to a past criminal offence no matter how old and how small. The American rule is that a customs officer has the right to deny you entry if you have a criminal record in Canada or the US.
Once denied entry you must obtain a US Entry Waiver (I-192) to return to the US which is a long, complicated and expensive application.
If you have not yet been denied entry to the US, then once your pardon is granted you will likely not be seen by the Americans and therefore will not require a US Entry Waiver (I-192).