Within an application, I've got Secret Keys uses to calculate a hash for an API call. In a .NET application it's fairly easy to use a program like Reflector to pull out information from the assembly to include these keys.
Is obfuscating the assembly a good way of securing these keys?
Probably not.
Look into cryptography and Windows' built-in information-hiding mechanisms (DPAPI and storing the keys in an ACL-restricted registry key, for example). That's as good as you're going to get for security you need to keep on the same system as your application.
If you are looking for a way to stop someone physically sitting at the machine from getting your information, forget it. If someone is determined, and has unrestricted access to a computer that is not under your control, there is no way to be 100% certain that the data is protected under all circumstances. Someone who is determined will get at it if they want to.
I wouldn't think so, as obfuscating (as I understand it at least) will simply mess around with the method names to make it hard (but not impossible) to understand the code. This won't change the data of the actual key (which I'm guessing you have stored in a constant somewhere).
If you just want to make it somewhat harder to see, you could run a simple cipher on the plaintext (like ROT-13 or something) so that it's at least not stored in the clear in the code itself. But that's certainly not going to stop any determined hacker from accessing your key. A stronger encryption method won't help because you'd still need to store the key for THAT in the code, and there's nothing protecting that.
The only really secure thing I can think of is to keep the key outside of the application somehow, and then restrict access to the key. For instance, you could keep the key in a separate file and then protected the file with an OS-level user-based restriction; that would probably work. You could do the same with a database connection (again, relying on the user-based access restriction to keep non-authorized users out of the database).
I've toyed with the idea of doing this for my apps but I've never implemented it.
DannySmurf is correct that you can't hide keys from the person running an application; if the application can get to the keys, so can the person.
However, What you are trying to accomplish exactly?
Depending on what it is, there are often ways to accomplish your goal that don't simply rely on keeping a secret "secret", on your user's machine.