`return` in Ruby Array#map

Sergio's answer is very good, but it's worth pointing out that there is a keyword that works the way you wanted return to work: next.

array.map do |x|
  if x > 10
    next x + 1
  else
    next x - 1
  end
end

This isn't a very good use of next because, as Sergio pointed out, you don't need anything there. However, you can use next to express it more succinctly:

array.map do |x|
  next x + 1 if x > 10
  x - 1
end

You don't need the variable. Return value of the block is value of last expression evaluated in it. In this case, the if.

def some_method(array)
    array.map do |x|
      if x > 10
         x+1
      else
         x-1
      end
    end
end

Ternary operator would look nicer, I think. More expression-ish.

def some_method(array)
  array.map do |x|
    (x > 10) ? x+1 : x-1
  end
end

If you insist on using return, then you could use lambdas. In lambdas, return behaves like in normal methods.

def some_method(array)
  logic = ->(x) {
    if x > 10
      return x + 1
    else
      return x - 1
    end
  }
  array.map(&logic)
end

This form is rarely seen, though. If your code is short, it surely can be rewritten as expressions. If your code is long and complicated enough to warrant multiple exit points, then probably you should try simplifying it.