On the PS3 it's possible to force DTS if you disable Dolby in the audio settings, but this requires stopping the program (or BluRay) - a little awkward. It would be nice to be able to switch on the fly, to make a decision about what sounds best for a particular source. PS3 software and the system software itself seems to prefer Dolby (if both Dolby and DTS are turned on in the settings, it always chooses Dolby). And if mult-channel LPCM is available it will choose that.
Some sources (movies for example) let you choose the format in the settings. Other times, both DTS and Dolby are present, but there isn't a menu option anywhere.