Actually, you can:
http://www.naftatnlawyer.com/applying-for-green-card-tn-st/
> It has to be renewed annually
There's a 3 year TN now:
http://www.naftatnlawyer.com/nafta-tn-blog/2008/10/16/three-...
If you're Canadian, I suggest subscribing to the Grasmick.com newsletter and poking around the site. Lots of good info there.
Not sure if the newsletter is up to date but I noticed that the most recent items on Grasmick.com's News section dates back to 2001!
The world of immigration into the US has really changed quite a bit since 2001, post-9/11 especially. For a start the INS doesn't even exist anymore and was replaced with the DHS.
So if the rest of his information is from this era and pre-DHS then it's pretty much worthless.
For some silly reason he's basically abandoned the website and what used to be incredibly helpful forums.
However, looking at the green card application process on the TN it doesn't look like it's any easier than it is on an H1B.
1. You cannot apply for a green card on TN visa 2. It has to be renewed annually, and you have to demonstrate continued strong ties to Canada (bank accounts, family, etc.), so US immigration would no doubt get suspicious if you kept renewing