From what it looks like, CWB is trading at a discount to the $52~ acquisition price.
This does create a 19% or so arbitrage opportunity. What I would guess is the market may think that the deal will fall through. This is a relatively big acquisition and there may be some regulatory hurdles to jump through before the deal is completed.
To be honest, I believe National paid a bit too much for the acquisition, but I'm still pretty bullish on it over the long-term. It gives them more exposure out of Quebec and National, being a larger player, can likely utilize Canadian Westerns assets a bit better and generate more profitability.
As per the arbitrage opportunity, it is an interesting one but one I will need to look into deeper before I make a decision as to whether or not I want to take advantage.