Some things have been somewhat leaked, but there aren't any leaks of screenshots or real gameplay details on stuff.
I think it could be mostly because it's real internal, 1st party stuff and really trusted partners who have worked with Sony for a long time.
I think Microsoft basically merging XBox and Windows officially and storing their conference details on a server which didn't have any password protection was a big reason why so much of their stuff got leaked.
I didn't know about Ubisoft, think Bethesda may have done a bit of their own leaking.
I'm most looking forward to Sony, their studios are well proven and a good 3-4 (maybe more) have been busy and quiet for a while, so I think we could get some good stuff if not great.
2nd and 3rd party partners could also have big stuff to reveal at Sony's event and they may have been great at keeping leakers on lock down.
Personally I think it's a case of either you do some soft leaks yourself and that's designed to build hype (I think Shinobi and Verendus on Gaf do this) or you just didn't keep your information secure and someone got lucky in exploiting that.
