Technically, even an "unpartitioned" disk has a single partition. When you look at the left-hand column in the Disk Utility window, booted from a single-partition internal drive with no external drives connected, you'll see two entries, the bottom one indented relative to the top one.
The top entry is the physical device. The indented entry below, which bears the name of your boot drive, is the logical volume, which—as you'll see if you compare the Capacity when it's selected with the Total Capacity when the physical device is selected—isn't quite as big. (In my case, a 37.3 GB drive contains a single 37.1 GB partition.)
The discrepancy is accounted for by the space occupied by the the disk directory—the structure which "defines" the logical volume (slightly like the way the exterior walls define a house, even one with a single room, and in so doing eat up a little of the overall footprint).
Running Repair Disk with the physical device selected simply runs Repair Disk in turn on each logical volume (partition) contained on that device. If you have only a single partition, there's no difference betweeen the two. (Of course, when you're booted from your internal drive, you can't Repair Disk on it anyway, but that's what the case would be if you were booted from, say, your installation disc.)
