I thought everybody knew this by now, but I have encountered two people recently who have been developing for the iPhone for a while and who didn't know this, so here it is. Sorry if I'm being Captain Obvious.
If you want to simulate a two-finger gesture in the iPhone simulator, hold down the option key. You will get two dots on the screen instead of one. The two dots will default to pinching - if you bring the dot closer to the center of the screen, the other dot comes toward the center, making it easy to simulate a pinch in or pinch out.
If you want to do a different two-finger gesture, get the two dots the distance apart that you want them to be, then hold down the shift key, while still holding down the option key. That will lock the position of the two finger presses together so you can do, for example, a two-finger swipe.
Still don't know a way to do three-or-more finger gestures, or complex two-finger gestures, but the use of the shift and option key should cover about 95% of the gestures you'd need.