That comes from the additives that improve drying time or prevent mildew. I'd only worry if I had long term / repeated direct contact with it in an un-"cured" state.

that is a precautionary statement for the solvents added as Ray said. They evaporate or "flash" and you will have nothing to worry about. True linseed oil is just the oil from flax.

Could I just treat the wood with flaxseed oil, then? Or would that cause problems with mildew, etc? I have flaxseed oil around because I feed it to my horses and goats :)

here's a great debate over which kind of boards are best. We think free boards are best -- recycled container pallets will do, or thicker boards if you can get them. Don't use pressure-treated lumber! It's got arsenic in it and other toxins, and no matter what they might tell you, it will leach out and get into your plants, and quite possibly into you too. There's another debate on how to weatherproof them. Don't use creosote! -- it's a plant poison. Paint or spray them with vegetable oil, or, better, linseed oil. Some linseed oil has toxic preservatives added -- check the label (or use food-grade flaxseed oil, it's the same thing). Or try this 100-year-old recipe for "Everlasting Fence Posts":

"Take boiled linseed oil and stir in it pulverized charcoal to the consistency of paint. Put a coat of this over the timber, and there is not a man that will live to see it rotten." (From "Lee's Priceless Recipes" 1895)
will do the same Uschi

It seems it also depends on what brand of boiled linseed oil you get. One can I saw at Ace talked about causing cancer, etc. But the can I bought at Walmart (Kwikstrip) had no such warnings. Weird.

Here is a link that might be useful: Garden blog