Mechman is completely right, George HW Bush went from having something like a 90% approval rating to being so unpopular that a 3rd party candidate actually looked like a viable replacement at one point. Carter took office with approval ratings comparable to Obama and the Republican Party in shambles thanks to Nixon and co's corruption and incompetence. Look how that turned out. And actually, if you examine polls both W Bush and Carter had better approval ratings overall around this point in their presidency than Obama. He's nothing exceptional in that department.
Now as to what I think would cause a landslide defeat? What I've been talking about the last few months, full scale economic meltdown. There's a lot of things which could trigger it ranging from commercial/residential real estate (we already have record foreclosures), the derivatives market bubble popping, bank failures overwhelming FDIC (we've had 81 so far this year), china's growing uneasiness over our debt, etc. There's also the Afghani/Pakistan and Mexican (Narco Gang) situations to consider too along with North Korea and all the other usual suspects.
But you're obviously a hardcore tool of the Republican reich wing my friend. There is no way in hell or hades that Obama is going to lose in 2012. He is the definition of political perfection. Hell, Obama is likely to win an electoral landslide of over 9000 votes. Because unexplainable fanboyism should have more impact on a candidate's realistic chances than right wing hackery (ie anybody who possibly entertains any possibility of a Republican victory in 2012).
But to be a bit more serious, I can kind of see why many Democrats consider Obama bulletproof right now. After all, the GOP isn't doing a good job of convincing people that they have sanes in their party. On top of what Mint just listed, the Republicans would have to find somebody who at least has a modicum of sanity to score a landslide election. If conditions do go to hell in a handbasket, anybody short of the mental insanity of Palin/Jindal could score a close electoral victory over Obama. However, the sanes in many states would prevent the mainstream conservative from winning even 300 plus votes. It would have to take a totally new brand of Republican to be able to score a 400 plus electoral landslide. There is only so many ways one can sell an ideology over and over and expect results. I believe this is what happened in 1984 when Ronald Reagan's brand spankin new (not really) brand of conservative Republican ideology triumphed over Walter Mondale's ancient cobweb brand of Democrat liberalism. The idea isn't really to abandon old ideas, but NOT TO SELL THEM LIKE THEY WERE THE FIRST TIME. Believe it or not but people can fall for the same thing again if you give it a different name. Hell, I'm shocked at how many people consider Ronald Reagan to somehow be a "libertarian" nowdays. If Ronald Reagan would be considered a libertarian nowdays this country is in a hell of a shape.
If people in this nation will buy that Ronald Reagan was a libertarian, there is a chance if the right Republican candidate came along with exceptionally good skills at bullsh*ttery and absolutely horrenduous at making verbal gaffes they could sell themselves as a whole new brand of Republican that is different than your "typical American conservative", even if they are your typical conservative. I mean really, just read this wikipedia article on one of my state US House Representatives, OK-4 representative Tom Cole:
http://en.wikipedia.org/wiki/Tom_ColeLOLWUT?
If Wikipedia could fall for this bullsh*t, so can the average American citizen.
But back to the point:
Obama is a very gifted politician, but I think it is unwise at this point to conclude that just because the Right is in shambles right now that there isn't a Republican who is just as equally gifted if not better at salesmanship than Obama. For all we know they could be hiding their good political skills to use in the event that the Obama presidency turns out to be a disaster so they could have a pretty damn good chance at winning election. If I had a choice to run for president after a failed presidency from my own party or a possible failed presidency from the opposition I sure as hell will choose the latter option. For all we know Matt Blunt could be a leading candidate for 2012.
Do not underestimate the cunnery of politicians ever. Of course if Obama is as gifted as many people claim, the aforementioned statement could be applied to him also. In summation, the whole idea of analyzing politics is a headache at the very least if not a full on migraine headache.