There is no way to do this consistently, I fear... It is system dependent: on WinXP, for example, or lower, it was quite easy, but modern systems try to prevent applications to steal focus unexpectedly. Which is a good thing (when it works!), as I hate loosing the focus when I navigate the start menu or when I type in a window (like I do right know), and continue typing in a window suddenly appearing, sometime dismissing it involuntarily as the result of a key shortcut intended to go to another window!

This can be demonstrated with a simple sketch:

1. void setup()
2. {
3. }
4. void draw()
5. {
6.   if (frameCount > 300)
7.   {
8.     frame.toFront();
9.     frame.requestFocus();
10.     noLoop();
11.   }
12.   background(0);
13.   text(frameCount, 10, 10);
14. }

On my Win7 computer, the sketch's icon just flashes to indicate the window requests attention.
There are also some interesting information in a StackOverflow thread.

Thanks phi.lho!

Good workaround, thanks for sharing!