In the middle of moving all those underscore delineated URLs to dash delineated URLs*, Google went ahead and announced the rel="canonical" scheme for defining the preferred URL for a piece of content. While it's not the biggest deal for me, other than the "/" vs "/index.php" question, for many people with more dynamic systems it's a big deal. Bravo to everyone involved as it's a really straightforward, easy-to-implement solution to a common, troublesome problem.

Google
Specify your canonical

Carpe diem on any duplicate content worries: we now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.

Yahoo!
Fighting Duplication: Adding more arrows to your quiver

Avoiding duplicates in the search engine index has consistently been a key concern we’ve heard from webmasters and site owners. Over the last few years, we have made significant strides in finding duplicates in our crawler and index algorithmically and provided webmasters with better tools for controlling these. Today we are announcing our support for a new HTML tag, the tag, which helps reduce duplicates by documenting the preferred URL form to access each page.

*yes, I'm done. Three days, a lot of careful planning and more regular expressions that I've ever written later, everything on my site is now cool like this ( http://www.drunkenfist.com/art/graffiti-art/black-book/index.php ) Dash heaven.