Has anyone fast travelled with their companions a couple of times and then find out their missing? Happened to me already, i hooked up with that girl from 188 outpost, fast travelled a couple of times then realized shes not with me anymore. Is this one of the bugs they plan on fixing?

reload previous save

Well apparently it was fixed in the lastest PC patch - they added markers to all of the companions. So I suspect it will be fixed in the upcoming patch for PS3 and Xbox 360.

In the meantime either go with Swarley's suggestion and reload an earlier save if you have one, or just continue playing and hope that the upcoming patch solves your problem.