well, as most of you know the PlayStationPortable have support for memory sticks. and you can get upto 2gb of storage on it (even more with hdd stuff).
and i thougt of this: maby the PSP can work as a HDD for the PS2. I know that PSP works with LaunchELF, copying files from it.
it would be kinda nice, since you can transfer iso files pretty fast from pc to psp. but you might need to format it as a ps2 hdd?
this is just an idea, i havent tested it and will not even test it since i have a old PS2 with hdd