According to Gregory Ryskin of Northwestern University, Earth's magnetic field lines are being dragged by ocean currents, and changing due to the oceans' own magnetic fields, which are generated by electricity flowing through the seawater. According to other scientists, however, not so much:

Despite the supporting evidence, the hypothesis has been met with strong objections from mainstream geophysicists. "Physicists should know better," said one when contacted by New Scientist.

Not everyone is entirely dismissing the idea out of hand, however; Imperial College London geophysicist Raymond Hide says that his water theory holds water, and suggests that geophycisists should be thanking Ryskin for his research. We're just wondering why the poles are moving if it's not because of the oceans...

Earth's poles may be tugged around by oceans [New Scientist]

Swedish scientist Stas Barabash (of the Swedish Institute of Space Physics), believes that comparing the amount of escaping ions from Venus (which has no magnetosphere), Mars (which used to have one, but no longer) and Earth proves that our magnetic field may be accidentally helping our oxygen ions to abandon us, writes New Scientist:

Taking into account the different masses of the three planets, their atmospheric make-up and their distance from the sun, Barabash compared the rate of loss of oxygen ions from each one. He focused on oxygen ions because these are the most abundant ions in the ionospheres of all three planets. He found that Earth lost oxygen around three times as fast as the other planets.

Barabash points out that a planet's magnetosphere will always be far larger than the planet itself or its atmosphere. This, he reasons, means that a planet with a magnetic field will absorb more energy from the solar wind than it would if it didn't have one. This extra energy would be funnelled down towards the magnetic poles, so molecules in the ionosphere above these regions could be accelerated enough to escape (see diagram). Barabash presented the results this month at the International Conference on Comparative Planetology at Noordwijk in the Netherlands.

Don't get too worried about running out of air to breathe, however; Barabash estimates that we're only losing about 60,000 tonnes of gas each year from an overall total of "thousands of trillions of tonnes."

Earth's protective shield is stealing our air [New Scientist]

Scientists have determined that the Martian magnetic field might have been maintained by an asteroid in orbit around the planet — an asteroid that eventually orbited a bit too close to Mars and crashed into the planet. According to New Scientist:

Now Jafar Arkani-Hamed of the University of Toronto, Canada, and colleagues say the gravitational tug of an orbiting asteroid may have powered a dynamo by pulling on the fluid in Mars's core. The team's lab and model simulations showed that an asteroid orbiting 75,000 kilometres above Mars could have maintained a dynamo for 400 million years, before the rock crashed into the planet and switched it off.

Other researchers in the field are skeptical, saying that an asteroid in orbit wouldn't have had enough energy to start up that dynamo in the first place.

Asteroid Switched of Martian Magnetic Field [New Scientist]