I'm sure they'll make links nofollow. It's not hard.

It's very easy, but doing so degrades the value of the site for honest users.

Since Jottit doesn't link to its users' pages, their rank in the beginning is 0, and so is their attractiveness for spammers. Once Jottit builds a catalogue of user pages, it's dead.

If they build a list of links to users' pages somewhere on the main site, which presumably has a solid PageRank (I'm sure it will after some time), that would automatically boost users' PRs as well, no matter what's on the pages. And so the spam flood will begin.

They have at least two negative CAPTCHA already set up: an hidden submit button, and a ROT13 copy of the submitted data.

Who cares on a site like this? You only see pages entering their specific url - nobody will actually see spam pages.

Well, there's two separate philosophies -- one says that enabling SEO is bad because it distracts from the legitimate use of the site and the other is Squidoo.

If the site grows big enough, some significant part of spam would be easy to filter out statistically.

Easy? Please enlighten me what's so easy about filtering spam these days...

The same way as GMail and other large email providers. From my experience with GMail, its accuracy is near 100% and their heuristics is mostly based on statistical data, e.g. they probably detect large volumes of mail coming from one IP and having similar links, etc.

Yes. One of the nice things about a smart hosted gmail system is that when spam happens, they will see a corresponding flood of spam reports coming in over a certain amount of time, so that would make it much easier to stem the flood of spam. I am sure there is quite a lot to it which we will never know, but it does seem to be the best out there.