More on the difference between likely and registered voters in polls

From Gallup:

The July 25-27 USA Today/Gallup poll shows Barack Obama leading John McCain by a 47% to 44% margin among all registered voters but McCain leading Obama among likely voters by a 49% to 45% margin. This difference between registered and likely voters indicates that now McCain voters are disproportionately represented among the estimate of those most likely to vote if the election were held today. This difference (in which Republicans gain among likely voters compared to registered voters) appears for the first time in USA Today/Gallup polls this year. In earlier 2008 polls, more Democrats than Republicans were engaged in the campaign and considered likely voters. This is generally a rare occurrence given that Republicans have historically been more likely to qualify as likely voters under Gallup's model (a fact that has been borne out in the real world as Republicans are able to win elections despite facing deficits in party identification or pre-election standing among all national adults). . . .

For those interested, the post also has the list of questions used to determine if someone is likely to vote.