It's not baby's breath. It's probably something in (former?) genus Eupatorium, which includes white snakeroot, and can be super confusing. This page has some hueristics you can use to figure out what it is.
(I would have guessed that it was a white Joe Pye Weed, but "[c]ommon names can be confusing as they are often locale-based.") posted by pullayup at 4:48 PM on September 17, 2012
Oops, I meant that to be "includes white snakeroot and white mistflower"--sorry, drlith. posted by pullayup at 4:49 PM on September 17, 2012
Thanks to everyone who has replied. I think oneirodynia has it - the images here look pretty close. I've posted a close up of the flowers if they're helpful to anyone in making a more certain ID. posted by ryanshepard at 6:45 AM on September 18, 2012
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posted by drlith at 11:55 AM on September 17, 2012 [1 favorite]