A project I am working on currently is hosted on a VPS with a Plesk control panel. When attempting to upgrade from RC3 to the final release the process kept getting stuck on the database check stage as it was trying to write to the web.config but it appears it needs to write a temporary file to the root of the hosting environment. At this point an exception was being thrown because within a Plesk hosted environment you can't set write permissions on the root only on files and folders within it.
The following post led me to a solution that gets you past the step in the upgrade process that requires you to manually update the web.config, no hardship but just a bit of a PITA and will hopefully be addressed in future upgrades.