How do you check if domains are available? I searched a little for a domain name API not too long ago, but all I found were things that required you to sign up (and almost always pay money, I think).
bustaname just uses a DNS lookup for an A record at the root of the domain. they say it's not 100% reliable but it's much faster than whois and i guess they don't want to use the zone file.
DNS is much faster. whois is usually rate limited and a lot of whois services have restrictions on automated querying. Obviously, whois is more accurate though.
So. When I need to test if a bunch of domain names exist, I first of all do a DNS NS lookup. If you get a result, you know the domain exists. If you don't get a result, then do a whois.
This sounds like a cool idea, but after looking at it, I think the odds that you'll come across something better than brute-forcing could produce seems a little low. I mean, let's look at the most realistic deleting names on the list today. I'm actually selecting the least nonsensical ones here (e.g., not the long list of names like "o6fg29ra.com"):
cafebrucenyc.com, dubai-companion.de, obamacoolbrother.com, raceto11.com, oasistaxi.com, wacko.us (this last one was actually the best I saw on the whole list)
I'm a big fan of http://www.bustaname.com/ - particularly the random generator: http://www.bustaname.com/word_maker