They are vicious little creatures. A couple of 'em took up residence in our yard a couple years back and they'd give me an earful and an occasional dive-bombing whenever we would pass by.
Actually, I just thought the mental image of a swarm of tiny birds chasing a fleeing little old fat woman in tie dye would entertain you. It was really kind of neat--there was only one bird, and it flew up, gave me the stink eye while hovering, and flew over and pecked at my arm a few times to see if it tasted as good as it looked, and repeated it a couple of times. We've had them come zipping in to hover right in front of your face (a little startling, because they're so freaking fast and they have that pointy beak right near your eyes) but there's really nothing to worry about. You might get a dive-bombing display like tk59 said, but they aren't going to really attack in swarms.
They do seem to be attracted to tie dye, tho. The hornets are, too.
I've always gotten a kick out of the teritorial nature of hummingbirds. I personally don't mind when they get a little aggressive, its kind of fun to watch and they really aren't going to hurt you (much buhwahaha). We run about 5 feeders up north at our cabin and that adds a little spice to the day when filling them. What you do have to worry about it the bears that like the feeders too. I will see if I can hunt up some of those pix this weekend.
The hummingbirds that sometimes show up around my house are rather friendly, it's odd. We have actually picked them up, had one land on me, we have held them and let it eat from the hand. We do not have feeders for them or many flowers. I have never seen one attack or anything similar, they usually seem to flee from people in most cases I have seen them anywhere else.
When I was a kid I sat and stared at one for a good 5 min. It can get pretty scary cause that beak at about 20 mph (well it seems like its that fast or faster) would be worse than getting stuck by a lawn dart.
Very beautiful creatures. They can have a very harmonious effect to everything around them.