From Montreal's side I take it as a sign that Halak's agent, who I'm sure many of you are aware of the reputation of, is playing serious enough hardball that they know they can't pay to retain him and have to get something in an environment where everyone else will probably be pretty sure they can't pay to retain him either. He's a long way from being a Blue too, after all - only a couple weeks to sign the guy before things get complicated. Getting something back is better than nothing and goalies are notoriously hard to move in today's NHL(see: Bryzgalov). I'd be mad if I were a Habs fan too, but I imagine there's some logic behind.
It's a definite upgrade in net for the Blues, though, who haven't gotten awesome goaltending lately. Halak's value is probably "too high" right now since so much of what the Habs did well was team defense he benefited from, but he's certainly better than what they'd have or what they could get on the FA market. It's worth nothing that prior to any signing they make of Halak they have the lowest payroll in the league, so they can afford to pay the Client's crazy price. The Blue's will probably have a serious face-lift this summer with only like 11 contracts left, and Halak will be a big part of it. Mason's contract expired - don't expect to see him back unless they can't sign Halak. This would be a really fun summer to be their GM, I think - they could be awesome or terrible next year depending on if he can retain the right guys(EJ, Kariya if he can get him at a Selannesque price, EJ, Perron, Halak obviously, Hensick), grab a couple impact FAs, which he'll have the money to do, and get a decent first year out of people like Alex Pietrangelo. Blues could be fun next year, and this trade is a pretty good start with them assuming neither prospect magically becomes a 40 goal scorer. Which is exactly what they need, by the way - assuming they all up Pietrangelo and resign EJ they're pretty set on D outside of maybe getting a better #4 guy, but that offense is still pretty sketchy. Kovalchuk to STL???