I was helping my sister with an issue with connecting to her school's email server from her MacBook last night and discovered something interesting. Her school uses a webmail.domain.com address for the front end for OWA which is just a CNAME to its real address - a .local address. This all worked fine until a few weeks or so ago - we later pieced together that that's when she upgraded to 10.6.5.
It seems that in 10.6.5, OS X can't follow that CNAME for some reason. I could ping the .local address just fine and the browser connected just fine, but when I tried to use the .com CNAME'd hostname, it would fail. The kicker? The dig utility seems to ignore all of this madness which made it lie to me, and that did not help at all while troubleshooting.
I was able to find an Apple forum thread about this with someone saying that rolling back the mDNS binaries to those shipped with 10.6.4 seemed to fix it, but it didn't help in my case (and some others in the thread). I personally don't use anything at work or at home using the .local domain, but I hope this gets fixed soon as it breaks a lot of Microsoft-centric networks as a lot tend to use .local for one reason or another..