Does anyone know how I can revert a contact import (via the upload contacts script included in the site). The process included merges with existing contacts.
To revert an import done with the import functionality at Contacts > Import Contacts, you will need to restore from your backed up database file. As far as I know, there is no automatic "undo" on imports.
To your point, it might be worthwhile to add a new warning to the import "Are you sure" verification message that pops up right before the import fires:
For example, "Are you sure you want to import now? Be sure you have backed up your database as there is no 'undo' for this." (Update: I submitted a patch so if the core team agrees this will be added in future versions)