aaudetat
We're doing resume reviews right now, so I've seen lots of resumes. There's plenty of job hopping. I wouldn't address it directly - that makes you seem defensive. (I have changed jobs a lot, but I swear it's for a good reason.) Instead, talk about the various opportunities you've had that have led you to change roles. Don't go on too much about it - dwelling on negatives is a common mistake.
You might also be able to make a triumph through adversity story out of it, depending on the details.
Thanks aaudetat, yes, I don't want to be seeming defensive. But if adcoms ever bring it up, I want them to say "ah, yes...that's why she made all those changes". If it doesn't concern them, then hopefully it will somehow mesh with the essay I write. I still haven't figured out how exactly to do that though, ie, fit it into one of the essays without using an optional one.
No, I don't have enough material for a triumph through adversity story - unless you count the lack of H1 B (visa) sponsored jobs during the tech downturn. I don't want to go there, because I know I could come across as whining, even though it affected just as many americans.