Well, it's strong enough, especially considering mercury's small size. The solar wind is what causes atmosphere loss without a field and it's strong enough to deflect it, so...

Not entirely, you will still have atmosphere escape even with a massive magnetic field. Atmosphere loss is due to the fact that a gas at a particular temperature has it's molecules at a distribution of speeds, and for each molecule there is a corresponding speed that is sufficient for it to escape from the planets gravity. Hence the high energy(high speed) tail of the distribution past this escape speed allows molecules to escape. Usually this will only occur at the edge of the atmosphere as molecules lower down are more likely to colide with other gas molecules and lose the energy to escape.

What the solar wind does is to help increase the number of molecules that have energies(speeds) over that needed to escape, and also remove the escaped molecules away from the planet reducing the chance of another escaping molecule impacting and loosing the energy it would need to escape. The other way to increase the rate of atmosphere loss is to increase the temperature of the atmosphere.

The magnetic field, or lack of it, effects how much effect the solar wind will have on the atmosphere of a planet. Also planets a large distance from their star will feel relatively little effect from the solar wind even without a magnetic field as the solar wind drops rapidly with distance.

I was incorrect about venus's magnetic field (i could have sworn it had one) but can you really call mercury's field strong?  Mercury has a pretty weak field (what, ~1% earth?).  Or is the strong/weak division more to do with how it deflects solar wind than magnitude relative to earth (the lunar field does not direct wind around it)

I guess its probably wise to avoid too detailed treatment of planetary science, when really, so little is known about earth, let alone the billions of other worlds in the galaxy.

Mercury has quite a strong magnetic field and Venus' is incredibly weak, so a magnetic has little to do with atmospheric pressure.

Since most planets in the solar system have managed to hold on to some atmosphere and water for the last 4 billion years I don't think preserving it will be a problem unless the mechanics get really screwed up.

The problem is that in the case of Mars "some" atmosphere is .59% atm, and that's the most atmosphere on a non-protected body! The only bodies in the solar system with a non-negligible (for a human being) atmosphere are either protected by their own magnetic field, or the very large magnetic fields of their gas giant parents. Now the Solar System also doesn't have any large heavy planets in the outer solar system (Venus/Earth or bigger) so I suppose it's possible that a large, heavy planet that was far enough away from its Sun may have an appreciable atmosphere. The problem with that is that both the solar wind and the heat from the sun scale down at the square of your distance from the sun. I am guessing that the only way to have both an atmosphere and not freeze to death is to have a protective magnetic field.

Stars already measure their luminosity and size, so that's one thing already in place. . .   I'd like more complex terraforming, especially water.   I love the idea of "ice pushers" who ferry the rings of saturn back over to your new lunar colony to improve the hydrosphere.   The only issue I can forsee is if atmospheres bleed off, then you might find water impossible to preserve and all games will become a race to find the most outlying world with no solar wind and hide there.   But with Steve's other foresights, I doubt he'd let it come to that point.

Since most planets in the solar system have managed to hold on to some atmosphere and water for the last 4 billion years I don't think preserving it will be a problem unless the mechanics get really screwed up.

One must be careful when talking about things like heat.  I would assume orbital "Chaff," or reflective particles, that scatter light of all wavelengths (including infrared) and prevent it from reaching the planet.  the particles we're discussing are orbital and above the bulk of the atmosphere, so absorbed radiant heat is essentially irrelavent at the planet's surface.  

Either way: anti greenhouse gas is nitrogen + magic technology.  I support magic.

Future terraforming redesign has been mentioned by steve, where things like magnetic fields and hydrospheres are important.  I would love to harvest cometary bodies, or iceworlds, of water to establish hydrospheres elsewhere.  Water management is a HUGELY important part of space colonization.  As it stands now, we can create a wet atmosphere, but I don't think it will ever rain out as a hydrosphere.  I'm sure any development in these areas will complicate the use of anti-greenhouse gasses slightly, and perhaps effective limits will be instituted as a result of changes to the design.  (its crazy i can do a 3000 degree C ball of useless, airless rock, with no hydrosphere or magnetic field, to a viable 0 cost planet in just a few years with AGH gas.)

As an additional side note, lack of a magnetic field should cause atmosphere to bleed off as a function of proximity to star and luminescence of said star.  So that 3000 degree ball of airless rock would quickly become airless again without hundreds of terraformers operating at full capacity.

I always assumed that Anti-Greenhouse Gas was just a large amount of black dust blasted into orbit to form a kind of shade for the planet it is a simple solution that is possible even with current technology let alone the technology that exists in this game.

Also "a cocktail of trans-newtonian molecules" sounds fair.  Your ships are going from 0 to 10,000km/s in a 5 second span.  Science be damned!