I have a published guide on this topic that you might find helpful. Please mark the guide (and this answer) as such if you do: http://www.avvo.com/pages/show?category_id=6&permalink=answering-interview-or-application-questions-about-prior-convictionsarrests
The short answer in your case is that you need to check the terms and conditions of your SIS. That said, it is usually the case with SIS agreements that, while you plead guilty to the offense on paper, the judge stays imposition of sentence pending your completion of the terms and conditions of the SIS, and if you do complete the terms successfully the charges are dropped (dismissed).
Since the charges are dropped, you are NOT convicted of the offense, so you could safely put "No" if asked if you've been convicted of a criminal act.
However, read carefully -- some also ask if you've "ever" been arrested. You'd have to answer "yes" to that truthfully. If they ask you to explain, you could simply state "I was arrested on suspicion of shoplifting, but the charges were dropped."
Answers to legal questions are usually fact-dependent; my answers are general and based on the facts as you describe them and necessarily includes assumptions. More specific answers can be provided only after researching the appropriate law and a comprehensive consultation in which all relevant facts are disclosed. Until both a retainer agreement is executed and we have a consultation, I am not your lawyer and you are not my client. Moreover, I cannot be your attorney unless you are located in Missouri, the only state in which I am licensed. If you are not in Missouri, please consult an attorney in your state for assistance, as my advice may be incorrect or incomplete. This uncompensated answer is provided for informational purposes only, as a free public service; any reliance thereupon is at your sole risk.
Contrary to what the other attorney stated, the charges on an SIS are not dismissed or dropped after you complete the probation. He is correct, however, in that you do not have a conviction if you successfuly complete the terms of your probation, and this is because you are never sentenced. However, you are still legally adjudicated and are legally found guilty. If your job application is asking if you have ever been arrested, pled guilty to or been found guilty of a felony or misdemeanor, or been adjudicated on any offense, the answer is yes and you will have to explain. If you are asked only if you have ever been convicted, the answer is no. The answer in this scenario always depends on the way the question is phrased.