I brought up this subject a few days ago. Only one native speaker of English answered my question and wrote "It is unthinkable for somebody to do a thing" structure is wrong. But for this web site: http://www.google.com/search?hl=pl&q=unthinkable+for?, I wouldn't be asking you now to express your outlook on this subject? Since there are a few titles on this web site (http://www.google.com/search?hl=pl&q=unthinkable+for?) in favour of "It is unthinkable for somebody to do a thing" structure, I am in two minds about this matter. Please express your objective opinion.
I sense that "unthinkable that" is more common, but I wouldn't necessarily flinch at "unthinkable for." All the same, in your own writing and speaking, I would suggest that you favor "unthinkable that." Also note that "unthinkable that" is often used with "should" or "would," as in,
"It is unthinkable that we should be forced to raise taxes on the poor."