A large number of whois queries would probably be the only reliable solution to check wether a domain is registered, and the (public) data about that registration.

Also, the central database (whois.internic.net) will only return you some of the data (such as the expiration date and namservers). If you need to get information about other things such as the registrant, you need to query the referral that InterNIC provides you (i.e. whois.joker.com, whois.godaddy.com, ...). So, you'll actually need two queries for most domains.

If you ever though about using DNS, forget it: it's unreliable.

Michele.