Thanks to everyone for their thoughts and well wishes. My seventeen year old is taking it real hard. He was 5 when we got Max and the two of them were always close. (Chris used to lay on top of him for years.) As those that own big dogs know, 12 years is awful old for a big dog (130-lbs) but still it doesn't make it easier. According to the vet, he most likely had a degenerating spinal problem the appearantly slowly deteriorated the nerves and feeling in is front legs, to a point where finally this weekend he could know longer support his own weight. It was either watch him suffer and slowly continue to deteriorate or do the humane thing. I still sobbed like a baby though.
I'm now worried about my cat. He is a year older than the dog was and the two of them were inseparable. It'll probably be a while though before we replace Max. We have a long trip planned in July and I don't want to leave a new dog alone for that long. Thanks again.