This answer contains spoilers… (view spoiler)[Yes, it's Jamie's ghost. It hasn't been explained in any book as yet, however, there have been quite a few "hints" as to Jamie's ability to see events 200 years in the future. I guess we'll have to wait and see what the "official" explanation is. :) (hide spoiler)]
Frances The author has said she will explain about the ghost in the last book. She is working on #9 now which may or may not be the last.
Megan I'm only on page 191 of the first book but from what little I know I assumed it was Jamie, yeah.
Lora Swafford Nobody really knows. But considering he was staring at Claire, and wearing a kilt. I'm pretty positive it was. I was always confused as to how he watched over her and loved her when at that point she had no clue who he was??