As part of the most recent firmware update V11.0.026,
you can now search for addresses without being connected to a network.
Just be sure to use a suburb/town name rather than a postcode.

This feature worked for some customers previously, however those of us in Australia always had to maintain an active network connection.

Note: You need to make sure you have the maps downloaded and saved to your memory card. On a new N95 there are some basic maps contained in phone memory. I would guess that these are non searchable. See the tip on downloading the latest maps.
Version number required is v3.0 w12 b02 (to find your version Maps – Options – About)

Firmware