According to NASA probes orbiting Mars and amateur astronomers who are not, a large dust storm is developing on the Red Planet. Dust storms are regular features of the Martian climate, and sometimes they reach incredible size. Sometimes, in fact, they envelope the entire planet. The current storm is not presently threatening either NASA rover now operating, but if it envelopes the areas the rovers are exploring, it could hinder rover activity. Because the rovers get their power by collecting sunlight through solar panels significant amounts of dust in the air would cut the power available.
Such huge storms would obviously have an effect on human exploration and settlement of the planet. All equipment would need to be constantly monitored and regularly cleaned. The ideal solution would probably be to completely seal all equipment from the outside environment, but that might not be practical.
Of course, if the terraforming of Mars is successful, soil should some day hold dust to the ground. Huge dust storms would become only an occasional reminder of the planet's wild past.