I don't think you're going to get a strong third party in the 2006 presidential election, because no such election exists.
As for 2008...
I can't immediately think of a reason why this pattern would necessarily continue forever. It has done so three times in a row, yes, but what happened before 1960? In 1956 and 1952, there also was no strong third party candidate. That's four elections that went by without one - it's not until 1948 that you get Strom Thurmond winning a few states. Then before that you had FDR cleaning up for four consecutive elections, and again in 1928 there was no strong third party candidate. You have to go back to 1924 to find the next one, which was Robert LaFolette. That's five in a row without a strong third party candidate.
My main point is that you shouldn't just assume that since a pattern has continued for this long that it indefinitely will. Maybe there will be one in 2008. But maybe not. There have been cases where you get a strong third party candidate two elections in a row, and there have been cases where you can go ten or twenty years without one. It all depends on the national mood and who the candidates are.
