A WebmasterWorld thread spotted, I believe, referrals from URLs that have _gwt_pg= appended to the end of the URLs. So what does the GWT and PG stand for?
Well, PG can be page. Where Google would append a page number to the URL for you.
GWT can mean Google Webmaster Tools.
This is all speculation and tedster seems to think the same.
I think it's a good guess that gwt="google webmaster tools". I don't make much use of GWT -- is there some way that you can tag specific urls from within your account?
I will track this a bit and update the post if I find out more.
Forum discussion at WebmasterWorld.
Update 2: Vanessa Fox has said it is not coming from Google Webmaster Central.
I'm not sure what those parameters are, but they're not coming from webmaster tools.