Good post Sophie. I too have a dog that shows reactive aggression in certain situations (Tasha), only if she feels trapped, and in my opinion, it is very mild. The main thing I have to worry about with her is when I am out walking on a leash and someone passes us the opposite way, particularly a man, Tasha will try to nip their heels as they go by us with her head lowered and hackles up. She also will bark/growl/hackles around strangers but as long as she is not leashed or trapped she will just keep out of reach and bark until they leave or sit down or settle.

She started showing this fear/mistrust of people outside her 'pack' at 5 months or so old. I had a consult with a behaviorist who felt she is really a good dog and watched her behavior melt away to puppyish curiosity when he laid on the floor and showed no threat to her. How many people would label her dominant? It couldn't be farther from the truth. Tail is tucked, body hunched down to make herself smaller, etc.

I took her to obedience classes for socialization. She got used to the people in the class and relaxed around them and had no problem, but would still react to strangers, mostly men who are the scariest. She's also always gotten along pretty well with other dogs.

So I did a lot of work with her, perfected her off-leash heeling. She is so, SO good. But when I go trail walking in an area with people, I put a basket muzzle on her. It's very light, plastic and doesn't bother her, and if a jogger suddenly runs by us and she tries to nip his heels, I won't have an incident on my hands, though generally I can sense her tension, have her sit, etc. if someone goes by us. There are always times of surprise though, I can't control someone's kid racing up to us for example, so I take the better-safe-than-sorry route to assure her teeth cannot so much as give a bruise or nick.

I'm sharing this to just show another example of what people may blanket under the term aggression as well.

And here is an older (bad quality) video I took to show Tasha's work in off-leash heel: http://www.youtube.com/watch?v=2XuBzP7fL7o
(and she was chubby then! I hadn't improved her diet as much yet )