Although I don't have the DS version, the PSP game seems to have some of the files from the DS for some strange reason. Anyway, there's a load of stuff lurking in the guts of the PSP game.
There's a cheat mode, which can be accessed by bringing up the alert menu while playing the game, then pressing the two shoulder buttons and Select at the same time. There's probably an equivalent on the DS, if you can figure out the button combination.
I've found a number of unused ships, although the Doomsday Machine is the only one really worth playing. It can be accessed by editing the game's ShipData and/or individual level files.
There are some unused levels as well, including the full E3 demo (two versions) and two unused missions (one for each campaign). These tend to be earlier versions of the main game's missions.
The last thing worth mentioning are mods. I've created a few of these by editing the game files. As the makers of the game seemed to be fans of the old FASA games, I had a go at making an updated version of the single nacelle Saladin class starship.
There's a ton more info on my site (see my homepage link).