I wouldn't look at it that way. Obviously the majority of apps have been covered already, but that does not mean those apps have done the proper job in capturing the market. There may be a dozen apps mimicking what you want to develop, but guess what, if those apps only have a total of 1k installs between them all, then it's an open market.
I've spent a lot of time looking at apps that just fail right from the start and never pick up on installs and apps that pick up right from the start and keep in going. The actual functionality difference between the two is nothing, it's the same exact app. I see this time after time, there might be 50 apps doing the same exact thing, but 1 of those apps is getting all the attention. It's not because that 1 app is doing it better, which sometimes is the case, but because the one app was actually marketed well by the developers.
Many developers have gotten on the iPhone app train since it's release, but the biggest weakness for these same developers is that they have ZERO marketing experience. They might be able to develop a kick-ass app, but if they don't even know where to start to get any sort of media attention on the app, they're simply fucked.
As I've read on a credible source and have seen myself, hundreds of apps are being released every single day. That's a fuckload. That alone needs to tell you that once released, you need to drop stacks on marketing the app.