There’s a thread here on the DNS Stuff forums that talks about this issue.
It looks like certain registrars are monitoring searches for domain names, then sending lists of available ones that people showed an interest in but did not register to a 3rd party who then registers them and attempts to sell them on for several hundred (or even thousand) dollars.
I believe GoDaddy is one of those implicated in this.
Some people have had some success by simply ignoring these registrations. In many cases they are not real. With the assistance of a complicit registrar (they were previously using one in Venezuela, I think) they place a lock on the domain but then release it within a few days before payment is necessary. If they detect interest in the domain (hits on the pages, e-mails about it etc) they will go through with the full registration process, pay the five bucks to own it, then attempt to sell it on for several hundred times that price.
DNS Stuff provides a WHOIS lookup that can be used to see if a domain is available, but it’s not as convenient as those provided by companies like GoDaddy. They do try to guarantee that the queries cannot be hijacked in this way though.
The best advice is: be careful. If you’re out looking for a domain and find a few that are suitable then buy them all straight away. The cost is low enough that you should be able to afford to overbuy domains which you later let lapse. Don’t give these people the chance to buy them out from under you.