Using microcontroller as oscillator for high voltage power suppy?
Analog purists may scoff at you if you want to do this with a microcontroller, but then a 555 may be beneath them as well :-). Yet a microcontroller may be a good choice for an oscillator application.

First, your circuit can be Damn Small™. A PIC10F200 requires zero external components and is even available in a 2mm x 3mm DFN package if you want.
It's still a few cents more expensive than an LM555, but the price difference gets ever smaller; prices for the LM555 aren't lowering anymore, while those for small microcontrollers are.

The PIC10F200 has an accurate internal oscillator, which will give a much more predictable signal (frequency and duty cycle) than you'll get with standard precision chicken feed around the LM555.  

And of course, if you're already using a microcontroller with a spare I/O pin you get the oscillator virtually for free, possibly even with crystal precision timing.  

Only possible drawbacks:
- you have to provide a programming interface on your board, or program it before assembly,
- an LM555 can work at higher supply voltages than a microcontroller; up to 16V vs 3.3V or 5V for a \$\mu\$C. This doesn't seem to be an issue here since the circuit works on 2 AAA batteries.