Really, not with the arduino, no way ;)
You are aware of AVR-USB which enables an AVR to act as a low-speed USB client device?
I wasn't. Thanks for the link.
Does this not mean it must be possible for an AVR to both receive and transmit the correct signals?
I only had a quick look at the USB-AVR page and I think you are right.
But on the other hand I can't really tell anymore what this thread is about ... It started out with a question about mimicing an USB-keyboard with the Arduino (not even sure if that is the right interpretation of the initial topic ?), to some sort of USB-signal catcher?/repeater? to creating a new USB-device!
My doesn't work statement applied to the repeater/capture thingy.
If you want to build some sort of standard device like your own USB-keyboard, mouse, rotary-encoder hardware etc. there are chips available where you simply add an USB-cable and some resistors/condensers and your keys : voila your own USB-keyboard, mouse, whatever.
If you want to take the hard road, or if you are in bad need of some really special features you could try USB-AVR and write your own Linux kernel driver (easy part) and Windows driver (hard part, you might end up in an asylum before you finished reading the docs of the Windows DDK).