May be i got you wrong... Please nevertheless let me explain the full procedure to update the software on the phone in the 'Debian Way', aka. X-Terminal.

The "apt-get update" command does just update the list of available packages from the repositories that are listed in the file /etc/apt/sources.list or the lists in the directory /etc/apt/sources.list.d/ . the command checks the list using the keys from the repositories. The nokia key did expire last year, because those keys are only valid for some time. Nokia chose to abandon the ship,thus the key was never renewed. The message is just a warning, not an error message. It has no blocking mechanics attached to itself.

The actual command to fetch the new packages to replace old or faulty packages is 'apt-get upgrade'. This will download and install the latest version of the programs you have installed.

The full procedure would be The first command to gain the rights to install, the second to update the list, the third to actually install the latest and 'greatest' version of packages.

All this has no implications to all the semi-broken methods to get Nitdroid installed on the N900. This is some mess of its own, but my knowledge about methods to install may not be correct any more as time goes on. You might need to check the nitdroid forums for their latest information.