I heard a statistic on NPR the other day that 40% of the worlds annual hardwood harvest is used for shipping crates and pallets. Some of it is not so bad though I doubt much is at the prime end of the scale. At the flooring mill I worked at the lumber packs arrived separated by 4"X4"X48" blocking to accommodate fork lifts. They were strapped to the packs so were always KD (I checked them).
I found SA mahogany, bloodwood, sapele, jatoba, liginum vitae, oak, cherry, walnut...majority were #2 common poplar junk. We got pallets occasionally made of QSWO, boy those staples and ring nails are a bugger. Most of the pallets were southern yellow pine or poplar with occasional oak cross members.
Its cool to recycle pallets but you do have rocks, nails, mold and other hazards to deal with. And more often than not the wood isn't of much use, but keep your eyes open and you may find some worth working for.