I have three Arduino Micros. One is behaving as expected - it starts the application program immediately after power is applied.
I tested the same at a second Micro and was surprized to see, that it is obviously first running the bootloader after power up, which causes an unwanted delay of several seconds for the application to start. My third Micro also delays to start. This behaviour is the same, either powered per USB (only 5V supply or from PC-USB) - or Vin. I have not access to the first board any more and don't know its revision, the other two missbehaving are R3.
From the specification, the Micro should start the program immediately, the bootloader should be initiated only after a reset or when initalized for programming via USB.
The reset pin is not connected, trying to keep the reset pin additionally puuled up with a 1k resistor as a test does not help.
After updating the bootloader from the current Arduino IDE 1.0.5-r2, the application sketch is starting immediately after power up.