Ah, the Oxford Comma. Interestingly, it was "invented" by the Oxford University Press, and thus is, historically, distinctly English, not American. It was introduced as the "house style" of the OEP. These days, it is the "house style" of the Harvard University Press, which explains its popularity in America, unlike in the UK where it has fallen out of favour.
Apparently (and I have no way of confirming this), it is common practice in American newspapers to omit the comma.
It is also known as the "serial comma".
It's interesting that you used [the Ayn Rand and God example] - I presume you've read about the Oxford Comma at some point. However, you've only pointed out the confusion possible by leaving the comma out. Of course, there is the other side of the coin, where the comma can, itself, cause confusion. The usual "test phrase" used to illustrate this is:
[bq]
"We considered Miss Roberts for the roles of Marjorie, David's mother, and Louise",[/bq]
Is Marjorie David's mother? Or are there three roles up for audition?
When it comes down to it, there is no hard and fast rule in either the US or the UK. Use or omission of the Oxford Comma is something to which you should apply common sense. Use it if doing so makes your meaning clearer; omit it if it makes your meaning ambiguous.