This is the only feature (currently) I'm really missing - I prefer testers not to be able to see other tester's bugs because they can just duplicate other testers bugs instead of doing actual tests..
Please add this.

From my expreience , since registration is free, my testers don't mind registering. I think the current functionality is OK - if you want testers to open bugs, ask them to register. It also gives the tester a more 'personal' touch..