WILL OBJECTIVE C BE NECESSARY FOR NEW IOS/OSX RELEASES? IN IOS

Of course, this depends very much on what you are planning to do. But I think there are a some points you should consider:

Performance

According to this SO question the performance is not as good as Apple promised (yet). But if you care about high performance, you should wonder if Objective-C is your way to go.

C++ Code

Very important is the fact, that you cannot use C++ code in your Swift code without having a Objective-C(++) wrapper:

You cannot import C++ code directly into Swift. Instead, create an Objective-C or C wrapper for C++ code.

from the Apple Swift Documentation.

Platform

Swift uses the same runtime as the existing Objective-C system and the idea of Automatic Reference Counting. So in my humble opinion a basic knowledge of Objective-C and the ideas behind helps to write better and more performant Swift code.

Existing Work

It will take some time that all frameworks, methods, classes, etc. have a pure Swift interface. Almost every demo project from Apple is written in Objective-C, too.

Conclusion

So I think, you will need some basic Objective-C knowledge in the future - maybe this changes over time. But it depends very much on what you are planing to do.

Learning either ObjC or Swift is not the hard part. Work through your fear: it only indicates new things to learn!

Swift is also very much not like Javascript. Don't be fooled by its surface appearance (in fact, avoiding focusing on surface appearance in general is a pretty good plan).

To answer the question though: I would expect it to be a few more years before you won't get a significant benefit from knowing ObjC; there's a LOT of existing code, examples, docs, etc... all in ObjC. But there's little to nothing in the system APIs that will require it once Swift 1.0 is actually out.

Is there anything I won't be able to do with only Swift?

Yep - if you need to support cross-platform or legacy code bases with C or C++ code there's no way yet (an not in the foreseeable future) this could be done with Swift.

I think, it's better to read about Objective-C, at least basic things in order to be able to read it, because if you are going to be an iOS/Mac developer, the chances are high that at some point you'll have to interact with Objective-C code somehow (for instance, you might need to take a look at some third party library code). Learning the basics doesn't take much time, Objective-C is actually a rather simple language. It's just not very pretty :)

The other possible necessity for Objective-C is a code optimization. High performance parts of code are often written in pure C/C++. You can't mix C/C++ code with Swift. The only way you can do it is by using an Objective-C/Objective-C++ wrapper for your C/C++ code. And that's when you're going to need Objective-C knowledge.

But actually it's possible, if I'm not mistaken, to use just Swift for development. So, my advice is the following: if you really hate Objective-C, use Swift as the main development language, but learn Objective-C basics in order to be able to read it (at least). Just in case.