Dogmeat died 2 times for me.. luckily I had saved the game recently, so I just reloaded it..
But indeed, he's VERY resistant and very helpful, the 2 times he died, 3 enemies attacked him at the same time and I had no ammo :/
I've been playing the PC version, so I don't know if that applies to the 360 version, but I was very impressed by how the game saves the location of everything in the map! You can blow a guy apart and throw each of his limbs in a different town, if you go back to each town 3 days later, they'll still be in the same place.. really.. I ditched some of my items in a room cause I had too much stuff in my inventory, long time after that, I was passing by and entered the room again.. for my surprise, all the items were still there on the exact same way I left them.. doesn't sound like something big.. but it is.. really (considering the anarchy in the world and all the raiders it's also kinda unrealistic, but still impressive).