Given that Poland has been the Welcome mat for every would-be conqueror for centuries, I'm sure this pushed them into a decision. I expect we'll see a sudden rush of applications to join NATO.
Not necessarily a good thing.
Why would the Russians be so against a strictly missle defense weapon if they were not planning on using missles on a country?
I don't think it's the missile defense proper that has Russia so pissed off. I think they're pissed that we're meddling in their corner of the world, that our stature and influence is growing while theirs is falling. I think that's part of the reason they pounded on Georgia the way they did, as a warning to other fromer Soviet nations that cooperating with NATO and the missile defense isn't something Russia will tolerate.
I think it's a darned good thing that Poland signed the missile defense deal when they did. It's a message to Russia that those tactics won't work, and that most countries would still prefer to side with the US even with the threats of Russian invasion.