Tim I would like to make my answer so short as possible here, while I am affraid that the discussion might turn out to a war against web standards again, and that will piss me off for good. :)
So here we go:
What do search engines mainly want to see when they crawl a page? From
my experience, the
"content".
What do blind and text browsers users prefer to see first? The
"content". More about this here:
http://www.usability.com.au/resources/source-order.cfm
Consider that editing documents with valid markup, doesn't mean that the documents are accessible or usable.
For example:
If using longdesc, alt tags, title attributes, etc, doesn't make a document more accessible or usable, when they are meaningless for the users. And such crap happens, when editors use them for keyword stuffing.
Result? Bad accessibility and bad usability. And Bad
SEO!
About the site navigation, which is a major factor in terms of accessibility and usability, so far I know it is a major factor for Search Engines too.
There is so much I can tell about all this, but definetelly not in the forums here. I had here so much bad experience by doing that, but at least Googles last update confirmed that me any my excellent team and site rules!
I prefer to start a discussion about this, in our own
discussion forums.
By the way. The key of my recent Google
PR success was:
Semantics! Don't ask me here though what that means. :) Better PM me, and I will try to explain.
Thanks.