The patch will be out at the end of this week.
Testing's pretty much over now. Some bugs did turn up over the last few days, but they were all minor and quickly remedied. Some really hard-to-spot typos were also found and fixed.
The program patchers are now nearly finished. There'll be one for Macs, Windows, and Linux. We designed them to be extremely easy to use, since people had lots of trouble with the old menu patch. The Linux version was compiled under Ubuntu, I don't know if it'll work with other flavors or not, but if it doesn't, you can probably compile byuu's tsukuyomi UPS patcher to do things manually.
I'm waiting on a few last lines from demi, but lots of tedious, final stuff is being done in the meantime, like the readme, the troubleshooting/faq page, prepping the server, some other things you'll see later. This will be the last update before the patch is out.
Before the patch is released, you might want to get everything ready ahead of time. You'll of course need the Japanese ROM, which we can't provide. You might want to hunt that down now, if you have the time.
You might also want to find yourself a GBA emulator ahead of time and get it set up. I don't know a whole lot about what emulators exist on each OS, but I do recommend VBA-M for those who can run it. It's a recent project and superior to the old VBA, which hasn't been updated in years. The VBA-M project site is here, and you'll want to get the most recent compiled version here. Of course, regular old VBA works too, but VBA-M is great.
Mac users, you may have trouble saving your game. The troubleshooting guide and the readme will list several ways to fix this. Your best bet might be to try VBA 1.7.2 rather than VBA 1.7.4. Fiddle around with the preferences too, see if changing the flash save size works. I'm telling you this now, because this will probably be the #1 problem report we get once the patch is released. So, if you have the time, try out the Japanese ROM and see if you can get your emulator to save.
For people wanting to play the game on actual hardware, see this thread. I was counting on some gbatemp people to help provide a detailed guide about flash carts/cards, but it looks like it's not going to happen. So someone on the forum compiled this quick and handy list. I don't know the first thing about this stuff, so I can't answer your questions. Also, just like with ROMs, I don't want to point people to specific places to acquire this hardware. Some simple searching should get you what you want, though.
(EDIT: The following list isn't complete, there are probably many other things that will work. The list below is just a list of what was confirmed to work in the above forum thread.)
FLASH CARTS THAT WILL PLAY MOTHER3:
Slot 2 (GBA) carts:
* EZ-Flash IV (note: the "Compact" version will not work)
* EZ-Flash 3-in-1 (requires a slot-1 cart)
* eLink 1Gbit
* G6 Lite
* M3 SD
* EFA Linker
* Flash2Advance Ultra
Slot 1 (DS) carts:
(These all require a GBA expansion, such as the EZ-Flash 3-in-1, to play GBA games.)
* M3 DS Real
* EZ-Flash V
* R4 DS
If you have questions about flash cart stuff, please see the forum thread I linked to above. That's where you'll find people in the know.
I gotta say though, the game looks absolutely great on actual hardware. It's nice on emulators, but the real deal is... wow. Here are some pics, though they're not great examples: GBA Micro 1, GBA Micro 2. Certain things in the game - particularly the musical battle combo system - will lag with emulators, just because of how emulators work. But actual hardware doesn't have that problem. You can probably get accustomed to your emulator's lag with time, though.
Anyway, all that aside, I saw that someone started a playthrough of EarthBound to kind of commemorate the patch coming out. That sounds like a really cool idea. I highly recommend it :)