it's just possible that you had malware on Leopard \[...] which altered your dns, and then Snow Leopard removed the malware, but didn't alter the dns back.
Snow Leopard's malware protection doesn't
remove malware, so installing SL would not remove anything, unless you wiped the drive clean and made it a fresh install. See my [Mac Virus guide|http://www.reedcorner.net/thomas/guides/macvirus> for information about how SL's malware protection works.
Very subjectively, rogue dns crops up on 10.6.x infrequently, but often enough to suppose that
something is out there, catching the unwary & not stopped entirely by Snow's quarantiney stuff..
Unlikely. There are ways around quarantine in SL, as described in my [Mac Virus guide|http://www.reedcorner.net/thomas/guides/macvirus>, as is a method for testing whether or not your software used for downloads is bypassing quarantine. However, the vast majority of Mac users are not likely to be both bypassing quarantine
and downloading (from a thoroughly untrustworthy site and voluntarily installing) a "video codec" for playing web-based skin flicks.
As I mentioned very, very early on in this thread, DNS servers themselves can be compromised. This can be fixed by changing to more trustworthy servers, like the OpenDNS servers. However, it is important to note that this problem has nothing whatsoever to do with the end-user's computer.
I really do wish folks would read some of the previous posts... we seem to be treading the same ground over and over and over and over...