We've found this interview on videogaming247 about 'Dog meat'. The dog many of you will remember from previous Fallout games. So, YES he is included in Fallout 3 (so that's one thing we won't have to mod in..) and even though they don't tell us where to find dog meat,  (All I know so far is, it is 'somewhere in the wasteland') at least we know he's there and they do tell us 1 important tid-bit of information. If the dog dies, there's no way to get him back. So save often .. as always :)

What happens if he dies?

“Dead,” said Hines.

Is there another dog?

“Nope. One dog. Be careful.”

Dogmeat can be sent to find ammo, food, stimpacks, and so on, but if you don’t think about what you’re doing with him he’s not going to last very long, from the sound of it.

“So obviously you have to be careful about where you send him foraging for stuff,” said Hines. “If you’re attacking a Raider camp, or something, and you’re running low on ammo and you say, ‘Go find me ammo,’ and he goes running through a bunch of Raiders, they can shoot and kill him while he tries to do what you told him. So you’ve got to be smart about where you send him off.”

Luckily, our canine friend isn’t necessary to the plot.

“It’s an homage to the original game to have a memorable dog that you can have with you, and it’s a way to give you a companion.” 

If anyone has found where to get dog meat to join up with you, please reply with a comment here so we can add it to this knowledge article.