Not to derail the thread, but I have significant experience with Modafinil, as I have a prescription for it--so I figured I should respond to the speculation above.
The most relevant interaction that Modafinil causes is due to its /induction/ of CYP3A4. This is consistent with macropsia's report of /reduced/ visuals with psilocybin.
That said, Modafinil is a strong substance and shouldn't be combined with anything that can cause tension or nervousness--particularly norepinephrine activity. It won't necessarily kill you, but I don't think it would ever be desirable to combine something like Modafinil with most psychedelics, which often have a certain degree of tension associated with them during the come-up anyway.
Something that could potentially be interesting would be the combination of 2C-C, a relatively relaxing psychedelic, with Modafinil. However, this is purely speculation, and generally speaking I don't feel Modafinil is a good choice to combine with anything.