Posts Not Working Suddenly - 404 Error

Hoping someone can help?

I was working on my WPMU last night - getting close to being done with everything ... then suddenly the posts stopped working. All Pages work and all post lists work (e.g. category pages) but to try to actually view the permalink for my post is totally not working - every time I get a 404 error now.

What changed - only thing I touched last night was updated pages and posts with All-In-One-SEO-Pack settings. Also I edited a template for Pages (not posts). That's it!

Since this issue started, I've tried the following:
1. changed the theme
2. created a new test post
3. created a new blog and its own test post
4. Confirmed .htaccess didn't change since last time it was working (it hasn't)
5. completely overwrote all public_html content with a version from 2 days ago that was working.

...None of these things has gotten the site to start working again. Anything thoughts?

Note - I'm running WPMU 2.9.2

Thanks!