May I have this dance?
But, regardless, it's an interesting alliance, since the Greens and Grits are giving each other some added legitimacy by doing this. From an economical sense, the Liberals stand to lose 18 grand a year from the $1.75 per vote they'd get in Central Nova (probably less since May would siphon votes off, they save the money it takes to run a campaign and they get the ginormous influx of...dozens of Green voters in Saint-Laurent–Cartierville).
So the real trade-off comes from the legitimacy aspect and I do tend to think the Liberals come out on top here, if only because it gives Dion's environmental platform an implicit vote of confidence and helps out the inevitable "vote Liberal to stop Harper" messaging we'll see the last weekend of the campaign.
The trade-off is obviously that it gives the Greens a bit more credibility. And everyone has a different opinion as to which party that hurts most.
[Coyne's got an interesting take, touching on the optics of beeing seen as moving too far left]