Supposedly the earths magnetic poles are always moving around, so when they "flip", not much will happen.
The great thing is, WE HAVE THE POWER TO EXTINCT OURSELVES! It's something like, 10% of the worlds total nuclear/bio weapons will cause enough damage to extinct humans. So the rats and cockroaches would rule, and have modified forced evolutions due to radioactive mutations! Dare to dream!!
Even not barring us killing ourselves off wholesale, statistically we'll go extinct anyways. The dominate species always goes extinct eventually no? Not likely to happen in 3000-5000 years though I wouldn't think. But who knows! We're one meteorite away from Deadsville (and sadly, Michael Bay lied, there's no way to prevent a massive meteorite from smashing the earth if one ends up coming our way) that's also a statistical enevitability.
There's two sides to the human evolution thing. Science is inhibiting natural evolution because of eye glasses and what not. At the same time, it's forcing evolution the same way dog breeders can force evolution of animals. Richard Dawkins talks about Russians breeding Foxes to be domestic, and within 20 generations (or years, I forget which), they started looking like dogs as opposed to foxes. Who know what we'll be like with science modifying us! Clones, introducing enzymes to make our brains work better, or to spruce up our immunity... could be awesome!
Or could make us all extinct faster... aw well, you only live once, might as will see what happens! :P