I just went through a method commenting out several lines using CTRL-/ (a.k.a. "Comment Line"). Along the way, I realized I'd commented one I didn't want to, so I typed CTRL-Z. When I did that, all the lines I'd commented were undone!
If it matters, I was editing a Grails Service class.
IDEA: 7.0.3 RC (#7718)
JVM: 1.6.0_04, 32-bit