Soldering from a keyboard to the guts of a gamepad requires finding a keyboard with metal contacts you can solder to. Most of the ones out now just have a couple of sheets of plastic with very thin strips of metal printed on them. There's a bottom sheet with metal strips that connect the keyboard buttons to the onboard chip, then a sheet of plastic with holes in it where the keyboard buttons are, and then another sheet with more metal strips. The three are sandwiched together and when you press a button, the top sheet is pressed down, it pushes ever so slightly through the middle sheet and touches the bottom sheet, registering the button press.
This is just one of the ways they do it now but most ways are equally cheap and there really isn't a way to solder to anything inside them.
Battousai managed to track down a keyboard with a PCB for the entire keyboard, so I have soldered wires to that. Ironically enough they are wired to a Keyboard Encoder, so that its a keyboard with absolutely no ghosting or locking, but the only buttons that work are the ones that will be used for gaming.
The wires could pretty easily be soldered to the guts of a gamepad, and I have considered doing that with another keyboard I found that has a PCB the size of the keyboard, but I haven't committed to it yet out of hope that there will be keyboard adapters.
They already exist for PSX/PS2 and Xbox, so the controllers I've wired up to keyboard encoders can work on any of those with a simple adapter. But not the 360 yet. Unless teamxtender makes one
the only problem i see with teamxtender or watever its called is the fact the microsoft is being really gay about adapters and such x arcade have been trying to get an "ok" microsoft since the release almost a year ago so lets just hope they can get an OK