Does Xcode support regions?

No, you can only fold code on various defined scoping levels in Xcode.

You can use little tricks to make navigating via the function menu easier, though.

#pragma mark

Allows you to create a grouping where the label following mark will show up in the function menu. If the label is a hyphen, a separator is inserted into the function menu.

Also, the following labels in comments will show up in the function menu:

// MARK:
// TODO:
// FIXME:
// !!!:
// ???:

Obviously since #pragma mark is not really portable, if you're building a portable application and need it to work with a compiler that doesn't just ignore #pragma directives that it doesn't understand, the comment-style mark is a decent alternative.

I am going to hell for this but here goes:

At the top of a given file, put

#define FOLD 1

Wherever you want to fold something, wrap it in an if block like so:

if(FOLD) {
 // your code to hide
 // more code
}

That will let you fold it away out of sight.

That won't work in the place you want it most, that is, around groups of functions or methods.

It may be useful inside a long, linear method with no internal conditionals or loops, but such methods aren't common in general Mac OS X UI code, though if you're writing some big numeric or graphics-crunching code it could help group things.

And the if(fold) is entirely superfluous. Just use the braces inside a method or function and Xcode will fold them.