[kibwen]

I'm no astrophysicist, but I'm under the impression that the effect of the magnetosphere is popularly overstated. Rather, the reason that Earth still has its atmosphere and Mars doesn't is that Earth has the gravity to hold onto it more tightly, and Mars is just too small. Hence why Venus, which also (curiously) has no dynamic magnetosphere, still has an atmosphere: it's 80% the mass of Earth (compare Mars, which is 10% the mass of Earth).

[heavyset_go]

The death of Mars' magnetosphere allowed solar wind plasma to strip away its atmosphere in the past[1]. Mars once had a much denser atmosphere when its dynamo was active, and the stripping of its atmosphere happened when the dynamo died.

[1] http://redplanet.asu.edu/?p=2035

[vkou]

Titan is ~Mars-sized, yet it has an atmosphere thicker than that of the Earth.

It may be safe to say that both factors are at play, here.

[jeffdn]

Titan is largely protected by Saturn's powerful magnetosphere, which does a lot to help, I imagine.