The Game Boy begot the Game Boy Pocket and the Game Boy Color. The Game Boy Advance begot the Game Boy Advance SP and the Game Boy Micro. The DS begot the DS Lite, the DSi, and the DSi XL.
So it's no surprise that the 3DS has already birthed a successor in the form of the 3DS XL, though it is ever-so-slightly startling that it's arrived barely a year after the launch of the 3DS.
Maybe that's just a sign of the times. If Nintendo continues on their current trajectory (and game consoles' lifespans continue to increase), the 3DS could have three or four more iterations before it's done.
Nintendo seems vaguely aware of this, as although the new, bigger system brings literally dozens of improvements to the handheld arena, it also comes with at least one fatal flaw.