After one day of spiking, I now know that a light meter app is possible on Android (at least on EVO). I wrote an app that can display illumination value on screen. The next step will be adding an exposure value calculator to do aperture priority calculation.
Although it's kind of using the same mechanism as incident light meter, the accuracy of the light sensors on normal smart phones is probably not good enough to replace a professional incident light meter such as Sekonic ones. I haven't done a lot of testing yet but I already noticed that the accuracy is certainly not good enough for low light situations. On my HTC EVO, the accuracy below EV 6 is doubtful and it certainly can't tell anything below EV 2.
With that in mind, I still think it could be helpful for day light metering especially for those who are using mechanic film cameras without any built-in light metering.
I will keep everyone posted here.