URL Structure - no .html extension

I read somewhere - I believe on this site - that vBSEO recommends that your URL structure not have an extension such as .html and to use a directory structure (e.g. yoursite.com/forum/thread/title/).

However I just finished reading a short and basic article in which Matt Cutts says that he recommends the following URL structures in order of preference:

long-haired-dogs.html
long_haired_dogs.html
longhaireddogs.html

How Google Really Wants You to Optimize Your Site

Does anybody have any article where Matt Cutts mentions a directory-structure URL and that it is preferable over a .html URL?

Any commentary? I'm curious. I don't use vBSEO but I'm interested in hearing what your thoughts are.

Hypertext Style: Cool URIs don't change.

extentions come and go. look at site.com/cgi-bin/script.pl from just a short time ago. they are LONG gone, and dead urls.
GOOD url's will NEVER change, regardless of technology behind it, thus, using extentions is not recommended.

/this/ will always be /this/ regardless of php, html, xsq, or jtc extentions it may or may not change to that drives it in the future.

Honestly, what I'm interested in is pleasing Google; not making the decision best-suited for long-term change. (How crazy am I for saying that?)

With that in mind, is it still recommended to use directory-structure URLs? Or do I take Matt's advice and use an extension?

I do not use extentions and google loves me. 88% of my daily traffic is google ref's.