Arent you afraid that you loose your momentum without made any dollar from your app? You can find clean way to do advertising on an iphone app (look at Twitterific for example)
Yes, we have several clean advertising plans. i.e - sponsored cases.
If you are not confortable with that maybe you could contact them offer them that to have an idea of what they are ready to pay to be in your app. Maybe the amount will change your mind :)
I dont mean to be disrespectful with my comment but I made that mistake to wait to long before monetising my startup because I always wanted more and more data before selling ads on it. I lost a lot of time (and money) for nothing.
Now that I think about it, another point that should have gone into the blog post is that "be prepared to monetize if your app goes viral in few days"
If you look at the top 10 apps in the medical section (or the top 25 for that matter), you'll see that they have been there for months (or years) on end.
My feeling is that the medical section of the app store is fairly different from the games section (where apps have a very limited shelf life).