From the pages of the Nintendo DSi manual…

Nintendos not really good at letting everyone know all those cool small features it builds into its consoles, but now that the system is out on the street we can finally learn some more about it. First up unlike the Nintendo DS and DS Lite the firmware is solely in Japanese while its probably not that hard to navigate however as the icons in the firmware are pretty self expanitory. More features inside. Another new feature, well one borrowed from the Gameboy Micro is the capability to adjust the brightness mid game by holding select and pressing the volume buttons, you can also music the sound by pressing down volume and the power button. Last but not least the DSi also contains a firmware update option, much like the Wii one. More as we have it.

Source: NeoGAF