Here I was, eating lunch, minding my own business, watching everyone freak out about Peyton Manning, and Zachary Levine tweeted this:
Brett Wallace hit a no-doubt HR to right, where the Astros wanted him driving the ball more. Astros lead Marlins 7-0.
This home run was off Wade LeBlanc, who had yet to give up a homer this spring. Furthermore, Brett Wallace has had 583PAs (90PAs against LHPs) in the Majors over the course of 2010-11, and hit seven homers - all against RHPs.
Looking at ML Splits (and Minor League Central for 2011), we can find that in 1383 minor-league plate appearances, Wallace hit 47 homers. 402 of those PAs came against LHPs, and he hit a total of 9 homers in those PAs.
Going through his MLB stats page, he had not hit a homer off a lefty in any Spring Training. So, as best as we can tell, today was the first time Brett Wallace hit a homer off a left-handed Major-League pitcher.