AFAIK Moombeam hit it right on the head. IF the earth speeds up shorter days, more equatorial bulge. I am not sure how fast it would have to go to cause and significant damage or perhaps fly apart, pretty fast for that.
If is slowed, days would be longer, if it locked in like the moon it would destroy life as we know it.
The earths rotation speed has nothing to do with gravity, gravity depends only on Mass.
If the orbital velocity were to increase we would move to a closer orbit, it would not have to be a huge change (1-5%???) for the effects to be noticed.
Like wise if the orbital velocity decreased we would move further form the sun, agian small changes as above would be noticed.
Another interesting point of speculation would be what would happen if the earth lost its tilt? Seasons would be no more. It may also have serious effects on the polar regions, which would no longer have the long day/night exterems. Not sure right off all of the effects, but we may loose the polar Ice caps, If Antartica and Greenland melt it would raise the water level, all sea level cities would be in a big hurt. That leaves Denver in the US as about the major surviving city.