Since Nintendo is taking its sweet time in releasing a Zapper-style Wiimote case, enterprising young modders have taken matters into their own hands. Witness this Zapper/Wiimote mashup in which the IR sensor has been moved from the Wiimote to the barrel of the Zapper. The buttons remain active on the Wiimote, which then attaches to the gamer's arm via an armband (not pictured).
It looks pretty awesome, and I'm sure it makes FPS games on the Wii a lot more fun and intuitive. Not to mention the fact that anything that uses the original NES Zapper is badass by default.
Wii - NES Light Gun [AcidMods]