First that symbol does not mean that the sim is locked it means there is no sim installed. There is no way the phone is locked. It is one of 2 problems
1 the sim is no good
2 the contacts on the n900 are flattened. you can try putting a piece of paper behind the sim or try to raise the little prongs. Try the paper first.

btw mido's image is pr1.3

edit if you cant get anything else to install but 1.1 upgrade the old fashion way ...through the application manager