So, I just used tin foil on a starter for the first time, and I can now say that I do NOT recommend it. Prior to this, I had only ever used airlocks, and without an issue, but with my new stirplate I built, I didn't have a stopper that would fit in a 2L Erlenmeyer flask.
Tinfoil does let CO2 out, and air in - but you know what else it lets in? Fruit flies. So I found my nice 1.5L starter swirling around on my stirplate with a fruitfly doing the backstroke in the vortex.
This required me to take an extra trip to the LHBS and purchase more yeast for the sulfited mead that I couldn't just leave lying around, and while I was there I made sure to pick up a proper stopper as well. For those of you who are concerned about the "one-way valve" effect with an airlock, I recommend that you use an aeration stone and pre-oxygenate your starter with pure O2 like I do.
The coffee filter and rubber-band idea also sounds like a viable one to me, but it does remind me of that old experiment disproving "spontaneous generation":
I'm not saying that it will happen, but it could, and thats enough to discourage me from using anything but an airlock. Filled with cheap vodka, preferably.