Well, since I don't really have much need for the modem, I decided that it may be a much better use of space to mount the USB ethernet inside the case instead of the base. This is going to take quite a bit of patience, not to mention some soldering skills.
1) Once you crack open the case on the SMC 2202USB, you'd realize that it's actually a very small board, much smaller than the modem. In fact, the modem is almost exactly twice the size.
2) The only thing that'll ruin the fit is the height of the USB connector, and the RJ45 connector. This is not a problem, because you're going to desolder and remove them anyway.
3) The phone jack on the I-O is the same exact size of the RJ45 connector if you rotate the RJ45 connector 90 degrees.
4) The hole for the outer ground on the USB connector on the SMC 2202USB is just large enough to fit a screw that'll go into one of the four mounting stand-offs for the modem. Don't worry, the metal stand-off on the I-O is also ground. It's actually good to connect the two using a metal screw. Unfortunately, because of the way the board is layed out, you can only use that single mount point. You can however pop a nylon standoff in the center hole of the 2202USB board to provide additional support.
5) The 2202USB board is actually slightly longer, but it isn't a problem because it'll fit underneath the stock heatsink. If you choose to mount the HD like Fred M has done, it would be way lower and you don't have to worry either.
6) You'll need to insert a washer (or two) if you're going to mount this over CN13 (the modem connector). This is to clear the height to the CN13 connector without clipping the pins. Even with the added washer, it'll still fit under the stock heatsink. You can also mount it on the other side.
7) You'll need to unsolder and remove the USB connector, the RJ45 jack, and one of the phone jacks. I would recommend the phone pass-through jack (CN18).
8) Remove the metal plate on the back on the RJ45 connector. This is because it's plastic underneath, and you don't want to short the motherboard when you crazy glue it. You can actually remove it entire if you want, unless you're planning to somehow wiring it to the ground.
9) The RJ45 connector is a 90 degree connector, meaning the pins to which you'll solder will be on the side of the connector if you glue the backside down. I would recommend that you making all the solder connections before you glue it down.
10) You'll need eight wires to reconnect the RJ45 connector back to the 2202USB board. Four are for the ethernet connection (1-2, and 3-6. 4-5 and 7-8 are not connect/used). The four other wires are needed for the LED status lights.
11) If the 2202USB board is facing up with the RJ45 connector facing you, pin-8 starts from your left-most pin. It's all through-hole, and there's 14 holes to unsolder all together, you'll have to be patient.
12) Once you've removed the RJ45 connector, solder wires to the appropriate connections. Glue (crazy glue or heatgun glue) down the RJ45 connector once you're done. I don't like to glue things to the motherboard, but it's late and I can't think of a better mounting solution.
13) Unsolder the USB connector on the 2202USB. There are six through-hole mounts. The two outer are grounds. The four in the middle are USB.
14) Looking straight down at the bottom of the 2202USB, with the USB connector facing away from you, pin-1 is the upper right pin. Counting counter-clockwise, till you end up with pin-4 right beneath pin-1. Pin-1 is ground.
15) If you look straight down on the I-O's motherboard with the heatsink on top, pin-1 of the USB connector on the MB is also on top. Count straight down to get to pin-4. Solder the appropriate wires to connect the usb port.
Well, that's it. There's actually also enough room to add another SMC 2202USB card internally and mount it the same way, but I'm not sure if you could wire it to the same USB port. I think you'll probably have to connect up all the USB components to the second usb port (described at http://www.geocities.com/rarose2/index.html). Maybe someone who knows USB a bit better can comment. This would be the ideal way to use the I-O as a router.