My personal suspicion has always been that the "old man" was Saruman, whose mind was so bent on finding out precisely what had happened in that area that he unwittingly projected an image of himself -- his thought, since what he is in uttermost truth is a being of thought, not flesh -- to try to see who was around that campfire, and if any hobbits might be there (especially the Ringbearer). When they addressed him directly, he suddenly realized what he had done, "stepped back" and vanished. My own reasoning is that he is, after all, a Maia, and travel via thought is natural for him in his natural state, but bound to flesh as were all the Istari on this mission, he might be able to do it only in thought -- what might be perceived by others as a "phantom" state. Moreover, he is intensely interested in what happened here, as was indicated by the fact that he actually left the safety of Orthanc to do his own reconnaissance (and was almost caught by the angry Ents). That it was a "phantom" has, in my mind, been supported by Gimli's unease over the lack of footprints at the beginning of "The White Rider" chapter. Legolas dismisses it because of the springy grass, yet he himself says that Aragorn could read even a bent blade -- yet none are mentioned, if I recall correctly.
Under this rationale, it's also possible Gandalf "projected" his own thought as well, also unintentionally; this might be supported by the fact that the "phantom" wore a wide-brimmed hat, as is also how Gandalf is described when he first meets with the three hunters in Fangorn. Either could work, I think; I just lean toward Saruman because Gandalf seemed very certain that they had not seen him. But he could be wrong.
My brain feels rather knotted, now...
