Any way to lock the internal screen with a password, so anyone trying to look around the phone will be faced with a password-required window ?.. it seems the E90 would be Vulnerable without this......
Password locking the inside screen
If you just want to put a password on the phone you shoud go to Tools --> Settings --> General --> Security --> Phone and SIM Card. Once there change the "lock Code" and phone autolock period, if you need the phone to lock before the autolock times out then you just hit the power button (even when the phone is open) and scroll down to lock phone
There isn't any point just locking the internal screen, as everything you can do and see on the big screen with the phone open, can also be done on the small screen with the phone closed. That's one of the 'killer' features of the E90 which has yet to be matched by any other smartphone.
So if you need security, the only way is to lock the whole phone as KroniK suggests.