We took it in. There are LOTS of stray cats in the area, and it would have been eaten. You can't keep a robin in a parakeet-sized cage though, because it's cruel. They're wild birds and need space to move around. So, we got a parrot-sized cage for it. We mixed wet puppy food and bread crumbs with water, and hand-fed it with popcicle sticks until it was better able to feed itself. Later, we added dry mealworms, fresh fruit and seeds (though robins aren't really seed eaters - they go for insects and fruit). We also let it out of the cage a lot, so it could stretch its wings and learn to fly.
Over time, it lost its down and became more adult-looking. It still had spots on its breast, but that's usual. I let it fly around the kitchen and living room, and it spent a lot of its time on the couch behind me. And, yes, I had a lot of cleaning up after it to do. It was amazingly tame, and would fly onto my shoulder and just hang out there while I did dishes or whatever. Robins love water and baths, so I'd fill a large bowl with luke warm water in the sink, and it would splash happily, completely drenching itself.
Today, after installing a feeder and a bird bath, we let it go. I gathered the kids, who had become very fond of the bird, and were really upset about it going away. We took it outside, and aside from one little flight around the carport it sat on my shoulder for about 10 minutes just looking around. Finally, it did fly off. Hopefully it will stay in the area and we'll see it occasionally.
I'm sad. It was the right thing to do, but I'm still sad.