It's hard to believe that as recently as April folks were talking about a 'permanent Democratic majority.' That was silly, just as talk of a permanent Republican majority in early 2005 was. The only thing in politics that I think we can count on as permanent is the winds changing.
John Kerry won moderates by nine points in 2004. Barack Obama won them by 21 in 2008. Ronald Reagan won them by eight in 1984. Those voters are highly variable from election cycle to election cycle, and unless we enter a period under Democratic control where there is universal prosperity, no concerns about national security, no major ethics issues, etc. folks will get antsy and the Republicans will get back in charge eventually.
I think we're much more likely to end up with a three or more party system than a one party system. Let's say that Democrats did hold the Presidency and Congress for 20 years- if progress continued at the rate it looks like it's going to this year some set of charismatic left leaning leaders would probably finally end up splitting off and bringing a lot of folks with them. If the Republicans got in charge forever and there was a similar middling pace I think the hard conservatives would end up splitting off and doing their own thing as well.
I doubt that's going to happen though- a competitive two party system is here to stay. What I'm even more confident of than that though is if Republicans make major gains in the House and Senate next year, then regain control of them and the Presidency in 2012 you're probably going to hear all sorts of commentary once again about the death of the Democratic party and the possibility of a permanent Republican majority. That's just as much a part of the cycle as the inevitable shifts in power.