Your reasoning does not explain why cron.php were showing undefined function errors in the logs. After MichaelH said those functions were part of http.php, I had to add myself the line include("http.php"); to the top of the cron.php file just to fix it. That got my blog working again, but not the posts.
This was before and even after it was suggested I try re-uploading those files using ASCII transfer.
Re-downloading the installation zip file again and re-uploading and restoring the database in phpMyAdmin and re-upgrading the database tables still did not fix the undefined function errors.
For a final release, it should not have happened. Simply put.
After two release candidates of WP 2.7, someone should have picked up that problem.
Heck, I followed the step-by-step instructions on how to upgrade WordPress to 2.7 to the dot, and I mean to the dot.
MichaelH actually had a viable fix in his suggestion above: change to the default WordPress Theme.
All it took from there now was to re-test all the themes that work and don't work in WP 2.7. Only plugin that's a little wonky is Simple-Press Forum-- some parts of the forum are missing images or incorrect colors because white-colored text is unreadable. And, this is in three of the twelve themes I have installed. Re-uploading Simple-Press Forum hasn't fixed it.
I'm sorry to be harsh in my criticism, but given the two to three pages of posted threads on this forum from users who are having issues from new installations to upgrades, the final release of WP 2.7 should not have had these problems to begin with. I was only expecting broken plugins but not missing posts, missing sidebar, database issues, and whatever else has been posted thus far.