Remember, Howard Dean was the clear frontrunner in late 2003 and then plummeted in January 2004. I don't think the same thing will happen to Hillary, but you can't rule it out.
The thing about Howard Dean, though, is that he seemed totally unelectable by anyone's standards save the far left wing of the country. They created enough buzz around him to the point that the everyday man who doesn't read much into politics thought he was the Dem's best bet right up until he lost Iowa and started screaming wildly about how he was going to go to all these different states. It was at that point his poll numbers started dropping.
The difference between Hillary and Dean, in terms of their stances in the polls at this time during their respective races, is that Hillary is far far far more electable than Dean ever was and people know this. Not to mention the fact that she has been building this momentum since the early 90s whereas Dean didn't start planning until 2002-2003 to run for president. Another difference is money, something Hillary has a lot more of than Dean ever did.
Barring some unforeseen circumstance that changes things drastically, I see nothing stopping Hillary from getting the nomination and ultimately the presidency. She'll probably still pick Obama to be her VP, so all you Barack lovers shouldn't have too much to worry about.
