It's weird, though - on another Pioneer spec sheet I was reading about
the max number of mp3 files 'per disk', and it stated 999. I wonder if
that meant actual physical CDs instead of USB devices.

Of more interest to me now, however, is the stated amperage supplied
by the USB port. I can take that figure, and compare it against the
power requirements of various 2.5" and 3.5" hard drives in USB
enclosures, and ultimately have a nice little hard drive, powered just
off the USB, chock full of stuff.

There's also issues of device scanning - the HU will scan the USB
device for tracks, in order to make them browseable in the menu. But
this can take some time, especially with complex directory structures.
Some people are finding that the unit re-scans the device whenever the
car is turned on - and some people find it happens only when they
attach the device.

I found a forum where people are wrestling with this right now - which
drives work, which drives need extra power, how to supply that power
via the cigarette lighter socket, stuff like that.

Kind of bleeding edge stuff, I guess - but I think it'll all be quite
a neat solution once I get the techy details sussed. ;)