absolutely should be usable without css. It should look like a nice 1996 web page with css off. :) I'm nerdy enough about this that I often hit CTRL+SHFT+S when visiting a page to see how it's put together (this toggles css on and off if you're using the web developer toolbar). When viewed in this mode, source order becomes really important. There is still a tendency to put HTML content where you want it to appear instead of where it makes sense in the document when viewed without styles (like screen readers, text based cell phones and most often by search engine bots). A few extra links at the top of your source, hidden when css is on, can make text based navigation really easy for people using non-css enabled browsers. Even in this new version of phpbb, there is a "skip to content" link.
We just finished
http://www.quizilla.com/ and we're pretty stoked on the text based usability of the site. Some one at QZ seems to have moved the page title to after that "skip links" but that still works ok. Two basic css 102 things to call out: the company logo, company tag line and main navigation are image replaced so that when you browse without text (or images - cell phones, screen readers, bots), the link and content are still usable (and indexable by google et al). Also, the main navigation is at the bottom of the page. For seo, you want the content as close to the top of the page as possible. the "The N | Quizilla Network" shouldn't be up there, nor should that ad. We got rushed... :) Sites should be: Name of site/page, skip links, search, CONTENT. This is only doable with purely CSS layouts.
2 long winded cents,
Will