As far as I know, the only exploit Riot has ever punished people for is the Anivia bug, since it was so obviously gamebreaking. I imagine they will continue that track record here, as there have been similar mastery bugs in the past they have not punished for.
To be honest I just started hearing of this particular brand today. Riot does have a huge interest in fixing exploits, and quickly. They're going to be a bit busy today with releasing the store, and for a couple days as things settle down and they handle all the numerous questions that come from that. If the mastery bug isn't already fixed by then, it'll be pretty close to the top of their priority list I'd imagine.
Riot does want to fix this (and will).
They will probably not punish the exploiters.
They're very busy right now with the store, hence the lack of acknowledgment.
EDIT: And a VERY CLEAR message from Riot follows this.