Active Record and non-numeric, natural primary keys?

Hi,

Are we supposed to use numeric, auto-incremented surrogate keys with Active Record, or are natural (intelligent), non-numeric primary keys perfectly acceptable (advised)?

There is an eternal debate on this subject (http://www.bcarter.com/intsurr1.htm), but I was wondering if Active Record was designed with the idea of "surrogate keys" in every table. That's what we see mostly in every AR tutorial...

Thanks,