Does Gingerbread use extra battery power to make calls when the phone has poor signal?
I would assume this is due to the cell radio firmware, not Android.  But I'm not sure.  In many cases you can flash older Modem firmware using Odin or Clockworkmod, so you could test and see if the Froyo firmware affects this under Gingerbread.  There should be a thread in the Galaxy S forum on XDA with the modems, I'll update this if I remember to look for it later.