well as in my views nokia n900 is another name of the word convenience because it always helps you to feel free while using it as in shortcuts on desktop , contacts , awesome widgets etc.
i dont know why but it clicked my mind that does it have any app or widget to lock the screen rather we use the lock/unlock buttons all the time ? :P

You can make such a widget yourself, with the Desktop Command Execution widget or the Queen Beecon widget. The command to be executed for screen locking can be found here:
http://wiki.maemo.org/Phone_control#...creen_and_keys

this is what I'm using:
http://talk.maemo.org/showthread.php?t=63131