I just recently upgraded to Panther (yeah!), and noticed that I was getting failed DNS lookups on 80% of the websites I visit -- but only the first time. I would enter in an address and get an error message about a failed lookup or no hostname exists. I would try again and it would work fine.
After much searching, I finally found an article (Mac OS X DNS Issues on obzorg.org) that described a similar, though not identical, problem. The solution presented there seemed to work, so I felt I could share it here.
I'm behind a Linksys router that uses NAT. What I did was open up the Networks pane in System Preferences application. Then I created a new location (called "DHCP Manual DNS") which was nothing more than a copy of the "Automatic" location (DHCP defaults). Then under both Ethernet and Airport (The "Show" drop-down box), I added 192.168.1.1 to the DNS servers field. This seemed to fix the problem. So now when I'm behind the router, I have the "DHCP Manual DNS" location, and elsewhere I'm in the "Automatic" location.
Hope this helps others who use macosxhints.com as their first stop for Mac info.
[robg adds: There's presently a really long thread on this same issue on our forum site. If you're experiencing the problem, please try this solution and post here (and/or on the forums) if it works for you. I haven't had these issues, despite being behind a Linksys box running NAT -- the only difference I can see is that I don't use DHCP and instead rely on self-assigned internal IPs for all my boxes.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004110517042638