I have actually installed garmin mobile XT on my diamond and it kicks butt; voice nav and all! Since there isn't any official way to install the gps software from ANY company as their program requires you to have a sd/micro card slot so you can run the the key off the card; the diamond has the internal memory and no you can't copy the contents of the card to the memory and expect it to work. So this require a little hacking of the software to get it to work
I got a lot of info and help from this site to get it to work:
http://www.noeman.org/gsm/pocket-pc-smartphone-zone/
It will be up to you to figure out the rest though. Do some google searches on garmin moblie on htc touch diamond and you'll be supprised what you will find.