I don't quite get their plan on this (which makes me think it's about as well thought out as their other ventures and attitudes on the intertubes).
First, at no cost everybody gets to read 20 articles a month, and also to browse the "home page, section fronts, blog fronts and classifieds," and read the Top News section on the Apps. I'm going to guess that this is going to be enough for a lot of people: "What are the headlines today? Ooh, let me read about this breaking story. OK, I'm done now." BTW, since section fronts sometimes include Top News stories, I assume links to these will be free wherever you click on them. For example, right now the top three stories on the World section front are the top three stories on the iPad Top News section. (Truth be told, I assume they'll goof this up and the links won't always be free if you don't click on them in the right place.)
Second they're charging extra to have access for both an iPhone and an iPad app. Huh?
Third, if you come to one of their articles via a search engine, blog or other social media link, that article will count against your 20 if you haven't hit it yet, but you will also still be able to read it if you have already hit 20. Search hits will have a 5-a-day limit. How is this not a giant hole in their plans? The truly devious could simply enter the title they want to read into their browser's search bar, and then click on the subsequent link in the search engine. A little more work, but not much. Or they could put the link into their own tweets or FaceBook or blog.
And I assume they're keeping track of this via cookies, which is another hole: a decent cookie manager can let a user easily switch identities and circumvent the monthly limit with ease.
The NYTimes should know better than anyone how its users reach their articles, so maybe they've got this sussed out. I just can't help thinking that they don't.
On a side note, I also don't get why their bloggers keep their blogs at NYTimes.com. Krugman and Silver and the others could likely do what Frank Rich is doing and not lose much readership. Since they don't seem to get paid anyway, what does it matter to them? If their readership does drop after the paywall goes up, will they stay?
(Gruber's had a few posts on this. Here's one.)
Edited to add (21 March): That didn't take long: Twitter feed of all NYT articles is now up and running.