If they're already planning a new Gameboy I doubt it will appear soon. Obsoleting the machine too fast will generate a backlash like the one Sega suffered after the MegaCD/32X.
That said, it is not all that unlikely that they're designing a new machine. The release of the Gameboy Advance was already delayed by at least a year as Nintendo milked Pokèmon so it wasn't a "current" design by the time it appeared on the market. Additionally Nintendo may have misjudged what people (esp. developers) wanted from the hardware. It has quite good 2D (almost identical to the Saturn in many aspects) but there are many 3D games already released and in development. Hardware 3D acceleration isn't an absolute necessity, but an upgraded processor and memory architecture would go a long way. A larger ROM-space would also bring both improved graphics and sound.