When I make a call on my cellphone (on a GSM network), is it encrypted?
For the most part they are encrypted, but not sufficiently enough to be considered as safe, tap resistant encryption. GSM uses 64-bit A5/1 encryption that is weak, to say the least. $15 phone, 3 minutes all that’s needed to eavesdrop on GSM call article from ArsTechnica covers it pretty well IMO, if you care to read more about it.
However, it also depends on what you mean by GSM. What I mentioned above is true for what we usually mean as GSM (2G, or second generation protocols). 3G are generally considered slighly safer (for lack of a better word):
 Unencrypted communication (flag
Yes, all GSM calls use an A5/1 stream cipher. It has been found that this is fairily easy to decrypt using a Rainbow Table. Additionally, the FBI admits to intercepting all phone conversations in the US.
So yes, it is encrypted, and cell phone companies don't give a damn about your privacy. If you care about privacy, you should use RedPhone and TextSecure which is an open source projects by Whisper Systems (now apart of Twitter).