Here's the interview transcript http://www.stephanspencer.com/archives/2007/12/17/matt-cutts-interview/
Some of the trust factors discuused in the interview were 1) Links that looked like they were certainly sold causes Google losing a little more trust in your website 2) The link neighborhood comes into play 3) Google hard-codes and says: .edu or .gov links are good. They have this 'sort' of characteristic that Google would trust a little more. You can have a useless .edu link just like you can have a great .com link.
So basically, as Matt stated, you ask 5 different SEOs what is "trust rank," you'll get 5 different answers. Hmmm...to me that's kinda like accounting - what do you want the numbers to say?