You are right. That's what the spec says. In reality, I've never found a single hub or host that actually does this. They mostly just dump somewhere between 300ma and 700ma right on the wire. The really advanced ones don't crash the all the ports on the whole machine if you short or overdraw a port. Actually, I think my macbook may get it right (i don't plug homemade usb junk into my macbook), but my toshiba I know for certain does not.

You've got a great idea though. A simple micro like a PIC or an arduino (atmel) could do this and get all kinds of neat data on the power flow as well.

USB Specs are available here: http://www.usb.org/developers/docs/

USB 2.0 in particular (10.9 MB zip file): http://www.usb.org/developers/docs/usb_20_122909-2.zip

USB data blocker - totally awesome idea. It could double as a protocol sniffer if it had a buffer.

See, a blocker and a sniffer would have almost entirely opposite functions. Both are cool devices that could be built from essentially the same hardware, so you do have a point there, but I wouldn't combine the two. Too easy to end up doing the wrong thing and compromising your data.

this is a great idea. market it to paranoid corporate IT for blackberries with sensitive materials.