Hi all,
I'm picking up a Micro today, and I see from the product page:
The Micro is similar to the Arduino Leonardo in that the ATmega32u4 has built-in USB communication, eliminating the need for a secondary processor. This allows the Micro to appear to a connected computer as a mouse and keyboard
What does that actually mean and how do / can I take advantage of that?
Alas, this getting started page is annoyingly blank.
You would use the Keyboard and Mouse functions:
http://arduino.cc/en/Reference/MouseKeyboard
Ah thanks, John: I hadn't seen those Leonardo items on the Ref Page.
From a practical point of view though, what do those buy me? What fancy stuff can I do to amaze my friends?
If / when you first start playing with keyboard / mouse support put long delays between strokes / movements. It is fairly easy to render your computer useless when the board is outputting a continuous stream of characters / mouse clicks. 
For what it's worth, I found the keyboard / mouse support on the Teensy boards to work a bit better.
JimboZA:
From a practical point of view though, what do those buy me? What fancy stuff can I do to amaze my friends?
Make a keyboard from children's blocks.
Make a remote control for Netflix (e.g. IR receiver that outputs spacebar for pause / play).
Develop a keypad to use for browsing the internet while you walk on a treadmill.
Nick Gammon has a "common phrase outputer" he uses when moderating. The first keypress outputs "Post your code.".
A friend on mine worked with a guy who played solitaire at work. Constantly. It drove my friend crazy. I built a background program that would screw with his game. Randomly turn over cards. Trap the mouse. All kinds of interesting / annoying things. Similar pranks would be easy with a Micro / Leonardo.
Make a Makey Makey... http://makeymakey.com/
Hmmmm, I see a problem there CB. I'll need to build a treadmill, first....
I saw that, but thanks for the reminder. I'll be very careful to start.
So that's the Easter Weekend taken care of.....