3Derp - about 50% there, parts all printed great, soldering this weekend.
For anyone who is just starting in with this as your first Rp2 project -- the raspberry pi is incredible, but one place where it seems it is always miserable (the e-z setups never work for me) is networking. Just so you don't get frustrated right out of the gate, after you flash your new RetroPie image onto your microsd card and fire up your RPi - do this (if you want to use a US English keyboard):
hit F4 to go to command line
type "sudo dpkg-reconfigure keyboard-configuration" enter (enter after each of these commands)
This will open a menu that will let you select your keyboard hardware and your localization options. Upon exit you will get a "update-rc.d:warning..." but your keyboard will now have the " and | keys where you expect them.
The Rpi2 has a power management problem - if you are using an Edimax or other wireless dongle there is a good chance you will have trouble connecting to the network or holding a connection. Fix it this way:
type "cd ../../etc/modprobe.d"
type "sudo vi 8192cu.conf" - this will create a file with vi, the best of all command line word processors.
type "i options 8192cu rtw_power_mgnt=0 rtw_enusbss=0"
then hit esc
then type ":wq!" This will save the file and exit vi
then type "sudo reboot" - the pi will shutdown and turn off and then turn back on and reboot. Hit F4 again.
now type "cat /sys/module/8192cu/parameters/rtw_power_mgnt" - if you typed everything correctly, this will return 0 (zero). If you didn't, this will return 1.
You need to enable the RetroPie-Setup script to be able to run it.
type "cd RetroPie-Setup"
type "chmod +x retropie_setup.sh"
type "sudo ./retropie_setup.sh" -- this should get you into the menu where you can configure retropie. If you can get online through the wifi config tool, great. It never worked for me - "cannot connect" errors even through I could see the router and I know the password was right.
If this happens to you, do this:
type "sudo vi /etc/network/interfaces"
make sure the content of that file looks like this (for WPA wireless, for older encryption use google to find an answer): -- ( vi commands: move to the line you want to change, hit i to insert, a to append, or x to delete. When you are done with a particular edit, hit esc to be able to move around the file with the arrow keys again. This will take some getting used to.)
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp
when you are done making sure the file contains this and only this. Then type ":wq!"
type "sudo reboot"
Once your pi comes back up, type "ifconfig" look to see if your wlan has an actual IP address (something like inet addr:192.168.0.1). If it does, great! you are networked! If it doesn't try rebooting again.
Once you are networked, now you can install packages, upload roms, browse and generally life is much better. The newer releases of Raspbian have some dhcpcd tool that tries to take over and do this for you, but it never gives me anything but trouble. This has worked for me a couple of times. Now go get building!
Many thanks to the tons of forum posts at github, Modmypi, retropi.org.uk and others for this info. It really shouldn't be this hard, thank you to those who took the time to post their experience. I just collected it here.