I have the same problem, I am out of warrenty over a few months and I am receiveing the same treatment that you are too.
I traveled around 50 miles on sataurday to get to my nearest Apple store for an appointment with a so called 'genuis' @ Glasgow - Buchann Street - with the hope that I would get it fixed.
I told her about the problem with the mic, what I've read online and what I've done so far to try and clean it. She never even attempted to try the mic, she just took my word for it and her response was bascialy 'Because they cant open it up, the only thing she can do is if I give her £199 pounds she will give me an replacement'.. So her genuis technial support was to replace it. Fantastic News!!!
Furthermore, after I refused this offer there and then in front of her. She offered to 'air compress' clean the mic, however I again refused. Informed Miss Genuis that if there is dust / dirt, I was it out now air compressed back into it.
I have tried the vacuum trick, however it did work but only for a very brief time. It wasn't working -> Then it was crystal clear -> second attempt to see and it wasn't working again. Just complete noise, If I blow into the mic I can see sound waves on my app and If I shout into the mic close I can faintly hear myself.
Ohwell !!!!!!!!!