Oh, not so mysterious - I'm sure I've discussed this in the past ... there was no one source of anonymous tips, really.  They came in from a bunch of people and there wasn't really a pattern, other than maybe one guy who did a little more than the others (this guy's forte was correcting my erroneous assumptions or faulty info - never gave away any truly juicy stuff).  
I can only remember two or three cases where any kind of scoop, anonymous or not, got me in any kind of trouble: 
1) The rumor that the setting for the Oriental Adventures rulebook would be L5R's Rokugan.  This came from a genuine anonymous scooper I had never heard of before and never heard from again.  WotC asked me to take that one down and even agreed to grant me some kind of exclusive info/interview if I revealed the scooper's name, but I had already deleted the e-mail and didn't remember.  Plus I pointed out that pulling the story would just make it even more suspicious and call attention to it, so we agreed I would leave it up but not post about that topic any more. 
2) Someone really did break their NDA when they gave me a somewhat juicy and somewhat ranty scoop about the new Forgotten Realms setting rules.  It was the first big scoop on FR and it immediately cast the whole thing in a pretty negative light.  I agreed that I would not post about that from NDA-breaking scoopers any more as I could see the point about not poisoning people on something before it was in a final form. 
3) Skuuuuuurge!  This was a scoop I pulled directly from WotC's Dungeon Magazine web page.  It had some awesome stat blocks for a troll barbarian and a human rogue, stuff we had never seen before.  And then I was asked to take it down, and they took it down from their own site. Apparently someone had posted something they weren't supposed to post.  That was a heartbreaker!
I got lots of NDA-breaking stuff over that first year or so and I often had to decide "oops, this is a little too close to breaking faith with WotC" so I did hold some stuff back.  But I am happy to say that no one ever fed me really significant chunks of the game ahead of time - I was in the dark almost as much as any other fan.  We were all in this together. 
