Situation:

I have BLS on the fiel
My opp. has BEWD
I atk.
He avtivates Mirror Wall
Do I get to reply the attack or does it run into the mirror wall and die because BEWD is on the field?

Why would you get a replay? A Replay would only occur when the number of monsters on their side of the field changed i.e. Call Of The Haunted.
Mirror Wall would be a bit pointless if you couldn't activate it in response to an opponent's attack.

In short, your BLS dies and your opponent kills himself laughing.

As stated, Mirror Wall DOES NOT cause any replay since it did not change the number of monster's on the defending player's (Your opponent's) field so your Soldier would be cut to 1500 ATK and suicide into BEWD and you'll take 1500 damage unless you chain with Waboku or something.

You know, I'm starting to think that if this keeps up, we're going to need a Sticky for Replays x.x

List of things to sticky

Replays
The Immortal of Thunder
Barrel Behind the Door
Skill Drain vs. whatever
Black Paladin vs. revival
XYZ fusions and how you CAN'T remove a fusion with the last piece
Scientist/Morph/Summoner/etc Fusions vs. revival

LOTS of stuff people keep asking about