This morning (04Jun07) I heard a callsign lifeguard 1870 while I spotting at SFO. So I was looking for a little corporate jet whom I assumed would be using the callsign. Instead it was DL1870 (SFO-CVG) using the call sign. At first I thought I just heard it incorrectly but then after departure when she was switching over to bay departure she first used DL1870 but after the readback from departure she stated she is lifeguard1870. Has anyone else heard of a DL flight using this callsign? Or does anyone have any info about why they used that call sign.