If I just want to know if a domain name is reserved; is it sufficient to use this command and see if any domain name servers turn up, in which case it's reserved?
host -t NS example.com
It's a lot faster than visiting http://www.internic.net/whois.html and typing example.com to get much more detailed results, which I'm not interested in anyway.
NS records are not necessarily required for registered domains. The whois service is your most reliable option.
Note that most Unix systems and Mac OS X have a "whois" command line program that is really quick to use:
whois stackoverflow.com
I don't believe that you have to have a DNS pointing to your domain. Even if you had to have a DNS set up, there is no assurance that the box acting as the DNS server isn't down.
Absolutely not.
A past employer registered theirname.biz solely for use on the internal network: it had DNS entries on the inward-facing network DNS server, but nowhere on the internet.
I'm not sure the trick was particularly essential, but "imap.theirname.biz" has the advantage over just "imap" that it's unambiguous if you're connected simultaneously to multiple networks (in the absence of deliberate foul play, of course), so you can just use all their internal DNS resolvers. Also the advantage over "imap.theirname.com" that once you know the convention, it's immediately obvious that it's a private server, and hence the reason you can't connect to it is that you forgot to connect VPN. There may have been other benefits to which I was not privy: I'm a coder, not an IT tech...
Various TLDs have differing requirements for whether name servers are provisioned or not. For example ".de" does require that name servers are up and running and correctly configured before they'll allow the domain registration to proceed.
The technical standards for DNS don't require it though, in fact there's nothing in the core DNS specifications to link together the registration of a name with its subsequent operation in the DNS.
Therefore, using whois is probably the most reliable method, with the caveat that you'll need a whois client that's clever enough to figure out which server to talk to for the domain in question.
That said, checking for the appropriate NS record is a very good shortcut to check that a domain is registered, you just can't use the absence of such a record to prove that it isn't!