Since USB uses a master/slave protocol (completely unlike PS/2), adaptors contain a USB slave to PS/2 interface. i.e. you can't just shove the PS/2 wires into the USB hole.
PS/2 is an international standard, so any device which uses it correctly (you'd expect all PS/2 keyboard to) should work on any given PS/2 interface.
Not sure about USB keyboards, I've only written PS/2 interface software before. You'd hope there's a USB keyboard standard. Though if Dell use a universal standard or not is another question, they seem to like propriatery everything.
So if you have a Amiga keyboard to PS/2 adapator, then you'll also need a PS/2 to USB adaptor to attach your keyboard.
Of course you can buy a USB board for the Amiga and connect the keyboard directly to that. Not sure what the limitations are though, perhaps the keyboard won't work in some games, or perhaps during early startup?
