IFFT Concept in OFDM
The first thing you must change for understanding why IFFT is used for OFDM is to forget about time and frequency domains. 

As the name states OFDM is a technique about frequency domain multiplexing. When you think in FDM you'll have several channels modulated with a guard interval between then to prevent interference. If the carriers are orthogonal you'll not have interference, but without the waste of band guard.

But how you can have this?

One way is to use several modulators, as in FDM, but with the correct frequencies. This is hard to accomplish.

If you will implement digitally the filter bank,and this is the correct way to view IFFT on OFDM systems: a transmultiplexer(TMUX), is equivalent to an IDFT. This is why you can use IFFT as a fast implementation for OFDM. You can read one of the books about multi-carrier communication for a better explanation. This paper
http://wsl.stanford.edu/~ee359/ofdm_bingham.pdf is also one that I like about multi-carrier modulation.