If you want it to be cross platform, having support for more than one input mechanism from day one is a good idea.
I would add to this, and say that even if you support multiple platforms, don't just reuse the same controls across the board. Some control schemes work well for some platforms, but not for others.
A superbly depressing example of this, is Battlefield 3. The UI controls are clearly tailored for console input devices, and it shows real bad. It is absolutely horrible to navigate through on a PC, and it is one my main gripes with the game, which is a pity.
So, tailor your controls to the platform, and everyone will be happy