The following information applies to the Mk.2 systems (female voice, non-widescreen, software 2-x/xx), although you can get into Mk.3 systems in a similar manner. I am not responsible for any damage you may cause.
Jack Shu posted some info on the bimmer.org E46 forum on accessing the hidden menu.
To do this, go to Menu, select SET, then hold down the Menu button for 20-30 seconds
On a related topic, to go into Radio Service Mode, Turn off Radio, Turn on Radio, Press and hold RDS (or maybe SELECT / Mode key, it's one of those) for maybe 10 seconds. Use the < and > to navigate, use the preset buttons to change values. Turn off Radio to exit. I do not know the functions available, so I'm not the best person to ask.
I don't have any info on any of the available functions, so you're on your own if you break something.
The Video module selection does not appear to do anything :(
I heard the necessary hardware for video (ie TV) isn't installed. I don't know any more about this. If you have any info on video or any other hidden functions please let me know.
I've taken some pictures of the screens.
Set Menu - Start from here. This is also how you'd check the software version (SW 2-1/28).
Service Mode menu - This is what you should see after entering the Service Mode
On Board Monitor screen
Click on Functions (note Version is also the same screen)
Click on Key Functions
Click on Brightness
Return, then select NAVIGATION / GRAPHIC ELEMENT
return, then select GPS
Click on Functions (note Version is the same screen)
Click on GPS status
Click on GPS tracking info
return, then select Sensor check
return, then select Telematics settings
You can actually input some of your own data such as Lic. plate and Color.
There is also a service mode for the radio. To enter service mode:
Turn Radio off then on, then hold down RDS button for about 10-20 seconds.
Use the < and > to go thru the menus, and the preset buttons 1-6 to change data, but this depends on the item. Sometimes 1 and 2 are used to change values up and down. I noted the following on my radio:
Serial Number B06xxxxx
Software Version 48-97 40
GAL 1
DSP 0
TP-Volume -9
Area USA
AF Off
I have no info on any of the items in the service mode.
To exit service mode, turn off radio.