Tom Haudricourt confirms that Lawrie will be one of the prospects going from the Brewers to the Blue Jays in exchange for the righty starting pitcher. Lawrie was the top prospect in the Brewers' system, and has been a target of the Blue Jays for a while. It's worth noting that Lawrie is Canadian by birth, and while that isn't the reason why the Blue Jays have coveted him for so long, it certainly doesn't hurt.
On his blog, Haudricourt talks about the deal as if it's already done, and while the name(s) of the other Milwaukee prospect(s) aren't yet known, it appears that the only remaining obstacles are the physical exams for all players involved.
Marcum missed the 2009 season recovering from ligament replacement surgery on his elbow, but in 2010 he was one of the better starting pitchers in the league. He isn't overpowering, relying instead on movement, command, and a broad array of weapons, including a devastating changeup.