They seem to be using a simple cookie based scheme.
Everytime I've hit the paywall, all I had to do was delete my cookies and history. I could then reload the same URL (from the same ip address) and get back in for free.
The paywall is ludicrously easy to circumvent - clearing cookies, Chrome's incognito window, deleting the &gwh=stuff from the end of each page URL where you hit the wall.
But after a couple months of doing this, I paid for access. I'm not a rich man, I don't have money to throw at things I don't need, but I felt that the value I was getting from the NYT's reporting was worth paying for. I can't buy a copy here (although I do when I'm in the US) so this is my way of supporting great journalism.
I wonder how many people like me are out there - circumventing the paywall with ease, reading as many NYT articles each month as they want for free, but eventually deciding to pay to support the paper? Or am I an edge case? What I'm getting at is maybe the paywall's deliberately easy to get around, so you realise what value you're getting from the NYT.
Everytime I've hit the paywall, all I had to do was delete my cookies and history. I could then reload the same URL (from the same ip address) and get back in for free.