Akkusativ nach 'für'
we've learned that you usually have accusative after the word 'für'.
When it comes to a sentence like "Was für ein Typ ist er?", how come we don't say 'einen Typ' and use the nominative state???
This is because in this case, "für" is not a preposition (which would take the accusative indeed), but part of the idiomatic expression "was für ..." (what kind of ...).
Here's a good explanation, with examples, on canoo.net, my favorite German-grammar site: http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-was_fuer.html?lang=en