Couple minor corrections...electors assemble in their state capitols and forward their votes to Congress, Elector slates are chosen *before* the election (remember, early in the process there were several Republican elector candidates who publicly said they wouldn't vote for Mitt Romney...I have no idea what states they were from, which puts into question their ability to carry out their stated wishes).
Some states have laws which say that their electors are *required* to vote for the candidate they were pledged for, but it is unknown if those laws would pass constitutional muster. Some states (Maine and Nebraska) have their electoral votes apportioned by Congressional district, and then the two "extra" (for Senatorial representation) go to the statewide winner. This actually happened in Nebraska last time, which cast 4 votes McCain-Palin and 1 Obama-Biden. The states choose the rules for how electors are selected, they could even go back to appointment by the legislature (used in South Carolina until 1860, and presented as a possible solution to the 2000 Florida mess).
There have been 4 elections where the "winner" in popular vote lost the election...1824, where General Jackson won 41% of the vote, 11 states, to Secretary Adams 31% and 7 states (two other candidates split the rest), John Quincy Adams was elected President by the House of Representatives, 1876 where Governor Tilden led Governor Hayes by 250,000 votes, but because of irregularities in the South, several sets of returns were sent. They threw the whole thing to a bipartisan commission, but one of the Independents selected got elected to the Senate from Illinois and had to drop out, and was replaced by a Republican. The Democrats from the South folded when it was promised that the Federal government would end Reconstruction, and Rutherford Hayes was elected president. In 1888, President Cleveland lost reelection after his first term in spite of leading Senator Harrison by 90,000 votes nationally, because he narrowly lost New York and Indiana, and of course, in 2000, Vice-President Gore led by 500,000 votes nationally yet lost to Governor Bush when Bush won Florida due to the Supreme Court ruling.
It is still possible, especially given Governor Romney's huge leads in the South, that he may win the popular vote and lose either narrowly or by a considerable margin in the Electoral College.