We've all heard about the people who have tons of credit cards - getting new ones for the sign-up bonus (25 to 50 thousand miles, typically), either never using them or charging minimal amounts and paying them off in full every month, and then canceling them before the annual fee is due.
Some people claim to have had over 100 cards in the past X years by doing this, and as a result have millions of miles to their name.
What impact would this have on one's credit score? I know that in general canceling a card is bad because it decreases your available to used credit ratio, but in this case you're adding a few, canceling a few, etc. - are there any negatives to frequently canceling cards and getting new ones?