Just in case you've got questions...
If you're confused to what it is - the M3 Sakura is a new flashcart and loader, developed by the M3 Team.
This Slot-1 flashcart is only available in Japanese. The loader was designed for the M3DS Real flashcart, but the team decided to release a flashcart for promotional purposes (thanks Urza). The Japanese firmware only works on Japanese M3 flashcarts.
However, the firmware has been hacked to where you can boot the firmware from an English M3 flashcarts (more info below).
Here's some images of the M3 Sakura firmware (in Japanese):
Using the unofficial M3 Sakura English firmware, the M3 Sakura firmware could only boot on the M3 Sakura flashcarts (of course), the M3 DS Real, and the iTouchDS.
Now recently, Rudolph (developer of GBA Exploader and Wi-Fi/Slot-2 Dumper) has found a way to run the firmware on any Slot-1 flashcart that use Micro SD cards for storage! Flashcarts with built-in memory (such as the Acekard R.P.G. or G6DS Real) cannot boot up the M3 Sakura firmware.
You can read instructions for runnning the M3 Sakura firmware on almost any non-M3 Slot-1 flashcart here and here.
Also, Urza will be reviewing the M3 Sakura firmware for GBAtemp.net soon.
There's much more information about the M3 Sakura flashcart, firmware, and more on Densetsu3000's M3 Sakura FAQ.
If you have any questions regarding the M3 Sakura, you can simply go to the M3 subforum on GBAtemp, ask in the M3 Sakura FAQ topic, or this topic if you'd like.