No, it is not OK to threaten the President of the United States with death. Not even in a Facebook poll. Even if you disagree with Obama, he is still the POTUS. The office of the Presidency deserves respect regardless if you do or do not agree with the policies of the office holder.
From Secret Service investigating Facebook poll on Obama:
The social networking site Facebook on Monday pulled a third-party application that allows users to create polls after a site member built a poll asking if President Obama should be killed.
The U.S. Secret Service, the agency assigned to protect the president, has launched an investigation, agency spokesman James Mackin said.
"As is usually the case, our vigilant users reported it to us first," Facebook spokesman Barry Schnitt told CNN. "The USSS [Secret Service] sent us an e-mail late this morning PDT asking us to take it down. At that point, it had already been removed, and we let them know."