In the case of cleft sentences with what the verb is used in singular. So is would be the right form.
***not a teacher***
♥♦♣♠ NOT A TEACHER ♥♦♣♠
Well, opinions might differ, but since I know of a rule prescribing learners to use the singular verb in a construction of this type, no matter what follows the verb, I couldn't agree more with you, Mehrgan.