I've bought a new drive (Samsung 840 Pro SSD), and upon connecting it disk-utility is showing it as having one bad sector. Is that reasonable for a new disk, or should I return it?
I would return it while you still can. In my experience when a disk starts getting bad sectors it doesnt last much longer. As @dave said this may differ as each person may have a different opinion but I dont think its worth risking it. I would return
I eventually found this page, which explains these seemingly contradictory results. All SSD's have the ability to recover from a certain number of bad blocks. Each hard drive is different, and initially manufacturers did not allocate enough ability to recover from bad blocks, giving early drives a reputation for unreliability and leading people to state that one bad block was a cause for concern. Eventually, most manufacturers wised up, and started producing drives with enough redundancy, making bad blocks less of a concern.
Given that this is a new drive, there should be many extra sector's available, so while having one bad sector raises the chance of failure slightly, it isn't enough to worry about. I would check your new drive in a month and replace your drive only if you see the number of bad sectors growing.
Finally, no matter how reliable your drives are, you should always make backups of your data.