The harddrive for the PS2 has nothing to do with it. When you hook the PS2 up to your computer via USB or Firewire, it acts basically the same as any other USB device... it just sits there. You will have to wait until someone makes a program which allows your PC to interface with the PS2 before you actually do anything.
In the future, the USB ports could manage file swaping between the PS2HDD and your PC\'s HDD. Once on the PC, you can hack and edit the files and do stuff like create new skins for Red Faction Deathmatches on PS2 and HexEdit saved games. Then transfer the files back to the PS2 HDD (Along with a few choice MP3\'s for the GT4 Jukebox) and you\'re ready to go.
Provided the right drivers and programs are there, I don\'t see why the PS2 couldn\'t link to the PC for some Head to Head Multi-platform action. There\'s going to be a lot neat apps comming for the PS2 once the HDD is out.. and some crazy programs comming from the underground programmers out there.