I did a DC test of this circuit from page 2:
I replaced the speaker with a 10R resistor, just in case I damaged it.
With the input grounded the current flow was virtually zero.
With the input at 5V the current flow was 100 mA (which seems to be the limit for the BC546 if the value for "Collector Current (DC)" is the correct one to look at (absolute maximum ratings, heh).
Replacing the 2.2K resistor with 100R gives a higher current (160 mA) but I guess the transistor won't be liking that, huh?
The capacitor doesn't seem to make much difference. With no input voltage, the transistor won't turn on, and current won't flow, whether or not it's there, am I right?
I measured 1.8 mA current drawn from the Arduino (input) when on, so that would seem to be well within the limits of a digital output pin.
So as a simple "square wave" amplifier (or indeed digital switch) are there any problems with the above circuit?