For the Series I models, I think Holden only released one or two upgrades of the Nav DVD.
The IQ Nav system uses a very similar format to some BMW and Volkswagen Nav systems, so there may be a possibility of hacking together updates from certain bits and pieces found on the web.
For anyone playing at home, the Point of Interest locations (Businesses, Long Lat, Phone Numbers etc), school zones, red light and speed camera location use the SQLite database format.
(Anyone that writes software for iPhone, iPad, Android should be familiar with this format).
It may be possilbe to export / import some of this information from something like the
OpenStreetMap project.
You can open, view and edit and play around with this data very easily by using something like the SQLite Manager Plugin for Firefox.
The voice / sound files are the same format as a number of other Nav systems. (The file names need to be changed and matched up, but you can change the Nav voice if you so desired.
Which leaves the Street Information Database that is the only thing that will be difficult to upgrade. (Though I dont think anybody has played around with this yet, so its still too early to say it is possible or impossible to do at this time).