With firmware v20.0.012 on a German blue 5800 RM-356 I have this problem which was not in the v11 FW. I did a successful, legal update with NSU v1.4.98 and restored my phonebook (but not settings) with PC Suite 7.1.18. The fault:
Display is turned off (standby mode) and then I press the middle, white menu key, a message appears faintly saying (in German) "Use the lock/unlock slider to free the screen". I can use the screen for some seconds but it fades again and goes blank (black). What drove me freaking crazy was that using the lock/unlock slider now lights up the red/white/green LED keys when activated but the display itself stays blank. The touchscreen vibrates when I hit it and I think the taps do function but I CAN'T SEE ANYTHING. Using the slider again will fade the LEDs and everything is off. Retry with the slider and the LEDs come but no display! The phone refuses to get out of this mode despite using lock/unlock slider many times and waiting 10 minutes etc. I have to turn the 5800 off completely and restart to get back to normal.
A hard reset *#7780# , or the corresponding menu option, did not improve this really.

Solution to what seems to be a bug in FW 20.0.012:
Press the middle, white menu key and while the display is still faintly shown, activate the lock/unlock slider BEFORE THE DISPLAY FADES AGAIN. This brings up to the display as normal but at half illumination. Phew! Waiting 10 mins sometimes allows the slider to work and bring the full-brightness screen.

This took days to work out the problem. If you accidently touched the middle, white menu key once when the 5800 was blanked (in your pocket?) then the lock/unlock slider only brings on the 3 coloured LEDS and no permanent display.
Does anyone else suffer from this? I though it may be happening when the battery was low but now it seems to be easy to replicate.
Nokia.. please fix this!

same problem and other related problem here. hope this is a firmware issue.

Same problem i've
I'm in Egypt, and my 5800 from outside Egypt, when i email Nokia Egypt thay told me that i've to wait until the new software update from Nokia, and it's not a hardware problem, so i think i've to wait..... :(

I have a same problem since I got it 2 months a go and tried to update from ver 10.0.010 to Ver 20.0.012, but the problem still there.

Just worry about the on/off slider will be out of service coz to many times switch on/off to get the normal screen from white blank.:bawling:

Hurrah, problem seems to be solved in firmware 21.0.025 from April 09. If you now accidently press the red, green or white function key when the screen was blanked, the advice message still comes but it does NOT prevent unlocking with the slider shortly afterwards. Wait for the white LED to fade and then you can restore the screen TO FULL BRIGHTNESS with the slider.

With firmware v20.0.012 on a German blue 5800 RM-356 I have this problem which was not in the v11 FW. I did a successful, legal update with NSU v1.4.98 and restored my phonebook (but not settings) with PC Suite 7.1.18. The fault:
Display is turned off (standby mode) and then I press the middle, white menu key, a message appears faintly saying (in German) "Use the lock/unlock slider to free the screen". I can use the screen for some seconds but it fades again and goes blank (black).

I never experienced this issue when my UK 5800XM was on V20 FW.

One question I always ask forum mebers who report problems after a fW upgrade is...
Did you RESTORE a BACKUP (PC Suite or handset File Manager) after the upgrade (or after the hard reset)???

If the answer is YES, the next comment is always...
NEVER restore a BACKUP after firmware upgrades - use PC Suite Synchronisation before and after instead.

All kind of weird problems happen if a backup file created from an old FW version is restored to a handset with higher (or lower) FW version.

This may not be the cause of your problems, but it's always worth repeating for those who are not ware of the risk with Nokia BACKUP files.