Where was this confirmed? I have a hard time believing that. As far as I know, Notch hasn't talked about it except for the possibility, and even then there were no specifics give.
A couple people on the forums said he said this on the Brocraft stream.
You'll have to excuse me if I don't find that too credible. I'll wait for the offical update/verdict. I know he was there, but unless i see a complete transcript, I'm skeptical, beyond that, It's not like he won't tweak it.
Personally I don't understand why everyone things SMP is going to be a clusterfuck of griefers. SMP isn't out yet, so what I predict will happen maybe be inaccurate, but so too, are those predictions. Personally I think on any server with competent administrations, when there are greifers (which will be few and far between.) they will get banned not shortly after they start greifing.
I definitely don't see a need for land-claim flags on say, a server like the Bay12 server we'd (Hopefully) have. Since It'd just be a bunch of us guys from here. Larger servers who serve a more generalized population SHOULD have suitable administration to deal with it.
Maybe I'm wrong, and maybe every third person who, might I remind everyone has actually PAID for the game (unlike creative multiplayer), will be someone with a desire to destroy indiscriminately for no personal gain whatsoever. Other than the satisfaction of pissing someone off.
I'd like to think a bit more positively than that!
EDIT: Plus to effectively destroy in SMP, the greifer would have to make a measurable amount of involvement in the game, to get the proper tools and what-not. It's not like some wouldn't, but your casual greifer isn't the kind of person to do that.