I found the following rules for the 2 majority parties:
According to Rule 9 of the Republican party (
http://www.gop.com/About/Rules6-10.htm) only the Republican National Committee is authorized to fill "any and all vacancies which may occur by reason of death, declination, or otherwise" for the presidential candidate. The committee re-assembles. According to rule 8-B, they must give five days notice for this meeting. Committee members "representing any state shall be entitled to cast the same number of votes as said state was entitled to cast at the national convention." They continue until a new candidate is chosen with a majority of votes.
Democrats have a similar process (
http://www.democrats.org/pdfs/charter.pdf). Article Two, Section 7(c) of the Democratic Party rules stipulate that "a special meeting to fill a vacancy on the National ticket shall be held on the call of the Chairperson, who shall set the date for such meeting in accordance with the procedural rules provided for in Article Two, Section 8(d) of these Bylaws." Article Three, Section 1(c) of the rules state only the Democratic National COmmittee may fill vacancies in the nominations for president and vice-president.
so...looks like the nominating committee would reconvene, the nominating process would start over again...with the current delegates casting votes for whomever they wished.
I suppose, since there is no constitutional thing about when the election is held, that if the death was near October or November, the election could be backed up a month.
Let's hope we don't find out what would happen for sure