It'll do one of two things I reckon...
Make the system and iTunes think that you Desire is an iPod, so that iTunes can sync with it.
Or, it will just read the library XML file and use the sync software to transfer the songs across using the data in that XML file.
I reckon it's the latter of the two personally. Remember all the problems the Palm Pre had with iTunes constantly rolling out new software to prevent that from happening? Unless Apple have changed tactics recently? Which I doubt they have...
Been reading reports that it will appear properly like an Apple device in iTunes when you plug it in, so that it can sync like normal, except it will be a HTC Desire
