Who was the old man?

[Inziladun]

Mansun

It could have just been an innocent traveller wondering at night, unconcerned by the appearance of the three hunters and minding his own business!

I'm not going to discount it outright, but that's simply too coincidental for me. JRRT doesn't have a tendancy to make throwaway references in his books, and as I said earlier, the appearance of the old man there and then, yet having no importance to the story, just doesn't ring true to me.
I think Radagast is out of the question for the same reason. He had played a very minor role in the story to that point, and that early on, and there was simply no plausible excuse I can think of that would have brought him to that spot on ME at that time.
There has been some good discussion here about this. I always just believed it to be Saruman and never gave it much thought until now.
The point of the clothing descriptions between Saruman and Gandalf regarding hats and cloaks has been brought up. I don't see that as significant. UT makes mention in the chapter (I think, not having my copy handy) The Hunt for the Ring of Saruman being seen by Hobbits in the Shire and being mistaken for Gandalf. And as others have said, it shouldn't have been much of an effort for Saruman to change clothes.