Since I now have confirmation below, I'll make this into a nicely-formatted table. I've also done a bit of research and found answers to a few unanswered questions:
(The voltages are all found from a quick Google. While I have done my best to verify any vague-looking results I found to ensure they are all accurate, I will not be responsible for any damage to your cartridges caused by an incorrect voltage selection. Use at your own risk.)
|Console(s)||State||DIY provisions||Production run||Firmware support||Slot||Voltage||Link|
|Super Nintendo Entertainment System and Mega Drive/Genesis||(Built-in support in Retrode 1 and 2)||None||Ongoing batches of production of Retrode 2||Yes||N/A||5V||here|
|Nintendo 64 (with controller ports)||Working||PCB layout and schematic soon||Ongoing production||Yes||Mega Drive||3.3V||here|
|Game Boy variants||Working||PCB layout and schematic soon||Ongoing production||Yes||Mega Drive||3.3V for Game Boy Advance; 5V for Game Boy and Game Boy Color||here|
|Nintendo 64 and Game Boy variants||Working||PCB layout, schematic||Previous sold out; none planned||Yes||Mega Drive||5V for Game Boy and Game Boy Color; 3.3V for Game Boy Advance and Nintendo 64||here|
|Sega Master System and Game Gear||Working||Wiring key, design and build write-up/rough guide (for original prototype); PCB layout and schematic soon||Ongoing production (without Game Gear slot, but space provided to solder your own on)||Yes||Mega Drive||5V||initial design | final testing | testing follow-up | production status | wiring key (for prototype)|
|Atari 2600||Working||PCB layout and schematic||Previous sold out; none planned||Yes||SNES||5V||here|
|TurboGrafx16/PC Engine||Working||Wiring key||None planned||Yes||SNES||5V||here|
|Virtual Boy||Working||Wiring key; design and build write-up/rough guide||None planned||Yes||Mega Drive||5V||final pinout | build writeup | part 2 | case - part 1 | case - part 2 | case - part 3 and conclusion|
|Neo Geo Pocket variants||Working||Wiring key; Japanese design and build write-up/rough guide||None planned||Yes||SNES||3.3V||Japanese writeup | Google translated to English|
|Nintendo Entertainment System||Considering||Will be PCB layout and schematic if completed||Probably if completed||No||?||5V||None yet (source: a post below)|
|Atari Lynx||Planning||?||?||No||?||5V||None yet (source: a post below)|
|Sega Pico||Working||Wiring key; design and build write-up/rough guide||None planned||Yes (ROM format identical to Mega Drive games)||Mega Drive||5V||German forum thread | Translated instructions|
|V.Smile||Working||Wiring key; brief build notes||None planned||No (but generic ROM format works)||Mega Drive||Unknown (5V?)||here|
Is all of this accurate?
(Just about to order my Retrode from the new store! Very excited!)
EDIT: Just ordered, but the addresses appeared as blank in the confirmation email (despite being present on the confirmation screen). I assume it processed OK, but you might want to sort that out so as not to worry people.
EDIT2 (2012-01-05): Added NES to the list, and Colecovision rumours I just stumbled upon. Changed to a nice table.
EDIT3 (2012-01-08, Matthias_H): Stickied
EDIT4 (2012-01-15): Added link that I just found to original design/build of the SMS/GG adapter
EDIT5 (2012-02-11): Added "design and build write-up/rough guide (for original prototype)" to SMS/GG adapter's DIY Provisions column, because there is one.
EDIT6 (2012-02-13): Removed Colecovision, added Atari Lynx.
EDIT7 (2012-02-28): Added Sega Pico
EDIT8 (2012-03-04): Added newly-posted wiring key for the prototype of the SMS/GG adaptor. Fixed Pico entry (had a duplicate column for some reason). Added "slot" field.
EDIT9 (2012-04-27): Added the just Game Boy and just Nintendo 64 adaptors, changed estimated dates.
EDIT10 (2012-07-28): Added the SNES/MD entry at the top; added voltages.
EDIT11 (2013-04-07): Changed the statuses of the plugins that are apparently shipping.
EDIT12 (2013-09-27): Changed statuses of plugins, add V.Smile.