to the argument.  Many people in support of K-Rod have mentioned that it is likely to be rosin residue from wiping his hat after grabbing the rosin bag.  I have conducted my own little experiment, taking a ball cap and a rosin bag.  I then proceeded to get as much rosin as possible on my thumb, and then with the rosin on my thumb I wiped the bill of my cap trying to replicate the mark as seen in the photos on the underside of K-Rod’s cap.  I could not successfully get a mark like we see on K-Rod’s.  So I then wetted my cap a bit to simulate a sweat filled brim to a cap.  And again, I had no luck trying to make a similar mark.  I must admit though, I wasn’t using an official mlb authentic game cap – so maybe my whole experiment was just a waste of time.

weren't there issues about Street's positioning on the mound and how that doesn't follow MLB rules?  

Cheating is just a part of baseball, and I'm not talking about steroids.  

It's like football - there's holding on every play but it doesn't get called.  In baseball, I'm sure someone is cheating every inning (stealing signs, watching where the catcher sets up, scuffing the ball, greasing your fingers, etc) but no one's gonna cry about it, except for Scoscia.

according to rotoworld.com

So its been admitted that its resin (pine tarry greassy sticky stuff) and not rosin ( gymnast white drying powder). And from watching the game last night this makes sense.
Though interesting enough I think it lead to being quite a "tell" for K-rod. before his slider he didnt do anything out of the ordinary. but for his fastball he would whipe his pitching hand on his leg (to clean it off i suspect?) then he would go to the hat and rub it on his thumb and from there to his fingers. Very sneaky indeed. Is it for grip or for movement? I would have no idea. its meant to look like a cap adjustment thats all baseball ocd stylee but i think he's gotten sloppy and now its just looks like goopin up his hands.
Anyone else notice some consistency with it? Its hard to tell on tv as they change the shot alot.  maybe my boy Mikey P noticed and squared right up on it. damn good game