Constantin:
As nice as shunt configs are, I prefer either power measurement ICs (like the Allegro gear) or current transducers (itead, Tamura, et al) To me, isolation is of key importance to Arduino longevity and ultimately user safety (depending on the currents involved). Allegro ICs are offered with current ratings up to 75A, several folk make evaluation boards for around $20 or less. The current transducers offered by iTead are even less expensive but inherently safe and isolate the Arduino 100% from transients, over-voltage, etc. And they're super-easy to implement thanks to the work that the openenergymonitor.org people have put into documenting their non-invasive power meter, including a Emon library that does all the heavy lifting for you.
Thanks for the idea. I'll be reading up on this tonight.