You are not alone! I had a half starved pirahna, aka ds1 - no matter what I did to try and stop him from doing it, it never worked for long. I was constantly after advice, followed him all round the place in case he met another child and he would try to eat them, tried time out, leaving early, you name it, I tried it. I used to take him to the local baby club, but the other mums all decided that I "wasn't doing enough about his biting habit" and that if I didn't do anything about it, we shouldn't come back. Really supporting.
This continued through pre-school, where the darling members of staff knew about his biting habit, and set him up on going for just 40 minutes a week. He soon learnt that if he wanted to stay longer, he had to stop using his teeth on the other kids. He ended up attending pre-school for full length sessions quite rapidly, and really stemmed the biting down. Even with this help, he still didn't stop biting every once in a while until 18 months ago (he's now six and a half!)
Is your ds2 gaining anything from the biting (ie toys, next turn on the slide etc)? I think ds1 used to bite because he was quite a way behind on his speech and he got frustrated (he had his own language until he was three, and no-one else understood him except me and dh!) It's only since he can say what he wants and get his message across that he's finally grown out of it.
You have to carry on with what you are doing - be firm, let him know that biting is a definite no-no. Do the same thing every single time he bites, and he'll learn cause and effect ("I bite, mum puts me in the high-chair/time out step/reins - I don't like that much.....hmmmm") It may take a while to sink in, but they WILL grow out of it eventually - I'm finally on the other side, and I thought it would never end!