Believe it or not.
According to Slashdot.org's Games section, those clever folks at Ahead Games have figured out a method to overclock the Game Boy Advance. By tweaking settings on the GBA's processor, they have managed to double the system's speed.
Of course, the real question is why anyone would want or need this extra power. It seems that it will affect your games somehow; the Ahead Games rep mentions that F-Zero will play faster, and presumably other games will be affected in a similar way. But the real applications seem to lie in playing homebrew software and emulators.
Ahead Games is currently testing the mod, and they plan to offer it soon as "GBAccelerator". Like any other hardware mod, it will void your system's warranty and could possibly cause damage to the inner workings.