Is namecoin a currency? It seems like a very cool way to decentralize a DNS table (name -> IP address, right?) But I don't get why it has an exchange value to BTC or USD... Is there a fixed NMC price to register a domain name? Is Casascius going to stamp NMC keys onto brass coins too?
Not really. According to the Bitcoin Wiki:


  Namecoin is an alternative distributed Domain Name System (DNS) on the
  basis of Bitcoin software.


In contrast to a currency, according to Wikipedia


  A much more general use of the word currency is anything that is used
  in any circumstances, as a medium of exchange. In this use, "currency"
  is a synonym for the concept of money.


It is clear that Namecoin is not primarily intended as a medium of exchange, but with a more specific design in mind. The fact that it can be exchanged does not mean that it is used as a medium to exchange value.

For a more detailed explanation about what Namecoin really is, check out the Bitcoin wiki.