I have been in touch with Earthlink and have identified the following:
Earthlink runs PHP 5.0.5, at least for the affected sites and for the testbed they provided me. We have identified a single line of code in WordPress 3.0 that causes a fatal error in PHP 5.0.5 due to a bug that the PHP team fixed in 5.1. It appears this bug also affects PHP 4.4.
I have committed a change for WordPress 3.0.1, to be released in July, to ensure compatibility. This of course will not fix any issues you may have with a plugin.
Earthlink has been made aware of the issue and they have begun to and will continue to apply the fix to any reported sites. If you are so inclined, you can apply the fix yourself. For the following options, you'll need to wait until after the nightly development build for 3.0.1 is refreshed (in the next 24 hours). After upgrading to a build with the fix, you will be running version 3.0.1-alpha-15359 or higher.
Option 1. Download the WordPress Beta Tester plugin, use the "Point release nightlies" update stream, and upgrade WordPress. This will upgrade you to WordPress 3.0.1-alpha. You can disable the plugin after.
Option 2. Download the latest build for WordPress 3.0.1: http://wordpress.org/nightly-builds/wordpress-3.0-latest.zip. You may replace all core files (wp-includes and wp-admin) to get all changes made for 3.0.1 so far. Or, to fix this one issue, you may also simply upload wp-admin/includes/post.php from the downloaded package to your server.
If you are so inclined, I would also contact Earthlink support and suggest you wish to run PHP 5.2 or greater. Hosts generally have a greater vested interest in upgrading their servers when users voice concern. There are also other web hosts out there.
I would also strongly encourage upgrading to WordPress 3.0.1 when it is released, and always ensure you are using the latest and greatest version of WordPress.
For reference, this is the Trac ticket I handled this issue in: http://core.trac.wordpress.org/ticket/14160.