If something happened to both the president and vice president, the line of succession (which was established in the late 1700s and has remained in force since then) says the next person to assume the presidency would be the leader of the US Senate. So, in the hypothetical situation you named, the person who would step in is Nevada Senator Harry Reid, leader of the Democratic majority in the US Senate.
Yes, Joe Biden would become President.
When the president dies the Vice President takes office. In present times if Barack Obama would die Joe Biden would become president.
biden
Joe Biden has never been president.
Joe Biden
Joe Biden holds this office at this date in 2011.
If Joe Biden was to become president, he would at least be number 45.
Biden became the vice-president on January 20, 2009. He is running for re-election in 2012.
If Joe Biden dies, Obama will remain President. If Joe Biden dies while Obama is President a new Vice President will have to be chosen by Obama.If your question was based on who would be President assuming both President Obama and Biden both died, that would be the Speaker of the Hosue, Nancy Pelosi.
Vice President of the United States
The Oath of Office - Vice President-elect Jo Biden, Jr.administered by Associate Justice of the Supreme Court, John Paul Stevens
If Obama dies in office, his vice President, Joe Biden, becomes the next President.