I don't know if it is true or not, but I have heard that some domain registrars will buy up domain names that you search for if you do not buy it right after the search, ie... if you go back later to get it it will already be owned by the registrar and will be selling for a premium price. There is a quick 'safe' way of checking to see if a domain is available 1. Open up a terminal window or command line application. 2. Run nslookup -type=ns domain.com and if it returns a list of nameservers, then the domain is not available. 3. If you have Linux, *BSD or Mac OS X, you can run whois domain.com. Some of you probably already know this, but I just thought I'd share.
Agreed, the direct whois lookup is the safest practice to check a domain availability. I always try this first when I have access to my Linux console. When this is not the case I use to trust NameCheap's whois service, never noticed a "magical" already-regged domain even days after my initial query.