[Legate of Amon Lanc]

I think the most important points have been stated - especially in the light of Zigur's quote, later reposted by Boro.

Maybe the only thing I would disagree on is that Saruman would have necessarily spilled everything out to Sauron. Yes, he would have, in the sense that Sauron had definitely strong will enough to pull the information about Istari from him, had he wanted to do so. I actually believe, in the light of everything we know about Sauron - and also in the light of the thing quoted by Zigur and Boro - that Sauron actually wasn't as interested in the Istari. There were a gazillion other, more important things to concern himself with (such as: can I attack Gondor now? Will its allies come? Does Saruman have the Ring? Who does have it, then? Any news from the North? Where are my Nazgul? Where are the Halflings? Where is the Shire? - going sort of retrospectively here...) than to learn who were the Istari.

I think Sauron would not engage in the debate "who are the Istari?" any more than he would in the debate "who is Tom Bombadil?". It probably wouldn't change anything for him. Either an enemy is a concern, or not. If some Maiar are stupid enough to go "cloaked" into Middle-Earth, they are probably not worth the attention, or: Isildur's heir, for instance, is much worse.

The only interesting thing that remains unanswered is actually what Mith has mentioned, regarding Sauron's and Saruman's original part in Aulë's "flock". Of course, for both of them, it had been a long time (for Sauron, a really long time), so maybe the connection to them was already quite hazy and clouded by the ages (literally) of experience in Arda. But it would be interesting if Sauron at some point wondered "hey, I wonder if that guy played during the first part of the Music next to me..." Then again, exactly those things had long time ago ceased to be of any importance to both of them...