I'm interested in using the Corona SDK but I'm concerned about lack of Game Center support. Is it possible to make the native calls? Or some way to expose native iOs functionality more directly into the Corona SDK?

Nope, I don't think so. We have to wait for native functionality or use OpenFeint for the time

It is not possible to do native calls or include additional iOS features. There is a limited amount of extensibility beyond that using the APIs included in Corona. For instance, you can use existing APIs to develop your own particle engine, twitter API, ad delivery mechanism, etc.

While Corona does not support Game Center yet, it is coming soon. For now, you can include OpenFeint and a plist that instructs it to pass achievements and leader boards to Game Center. Take a look at the GameNetwork API.

While Corona does not support Game Center yet, it is coming soon

Says who?

PeachPellen, Carlos, etc. (this topic comes up just about every day)

With that said, I've now been waiting for Game Center functionality for over 6 months and I share some of your skepticism... lol